I just wanna see what happens

Imma beat that fucker with another motherfucker

YOU MEAN YOU GAVE A MAGICAL PLATE TO THE HALFLING BEFIRE GIVING A DECENT WEAPON TO YOUR BARB ????

What is hilarious is that a normal plate is like 1500 gold. This should be enough for at least 2 magical base weapons

5e's balance point puts a massive premium on AC - it's very expensive to improve AC.
- ... okay ? What does this says about anything ?
- I often think about the fact that AC is intentionally so hard to increase but sometimes gold is necessary like a paladin (or good loot, but it's often expected that they'll save for plate), while a monk can increase theirs just by leveling.
  I don't really have any AC critiques over this, but I have issues with gold and exactly how much should be given out. I now know that to earn 1500 gp is about a level 6 thing, but I don't know why, or even where I learnt it, I could easily be off by levels due to giving out the wrong loot, or a PC who roleplays spending their gold is mechanically harmed compared to a player with similar vices from another class. I'm not even sure how I'd handle this differently, except the more I learn 5e, the more scared I get of gold and time.
I can see a situation where a campaign centers around making monsters feel threatening and intentionally depriving a largely martial party of magical weapon. I've never ran a ghost or werewolf that is actually a problem due to either a spellcasting party or I already offered magical weapons. I kinda wish I had, because it would make that item feel much more exciting.
Meanwhile there are items like cast-off armour that don't really offer a major mechanical bonus, or even just an artificer or forge cleric making their armour magical themselves.
- Well... just make it invulnerable to magical damage then. Only weak to normal damage. Bingo, fun problem to solve.

Question from someone, who never played a Pen&Paper RPG. How do you handel obscure things as a DM? Do you make up the rules on the spot, or is there a database of obscure decisions, or is the rulebook exhausting enough to include this kind of stuff?

As a DM, ordinarily there are no rules for using fellow PCs as improvised weapons or for attacking incorporeal creatures with magic items other than weapons. However, there are a few things to consider with this approach:
If a player is reduced to doing this, they likely do not have any other magic items at hand that would make suitable improvised weapons (An aside - how do you have a party member with magic armor and another with no magic weapons or implements that might be used as weapons? Maybe follow this combat up with a discreet review of the PCs magical equipment and equalize if there's a large discrepancy)
it's funny as hell
Based on these two facts, assuming 5e rules, I'd let it work as a two-handed weapon attack, but the attack is made with disadvantage because the plated halfling with magic armor was not properly balanced for combat, and if it connects, it deals 1d4+STR mod damage to the ghost, and half of that to the halfling.
Step 1 : ask a player if they have any ideas Step 2 : do a google search for about 30 seconds to see if you can find something easily Step 3 : Rule with your guts, take a note of it to check between session, and MOST IMPORTANT STEP say this : I’m going to rule this like that for now and I’ll check in between sessions for a correct ruling. Do not use this here today as a final conclusion in a later session please.
- 100% this. The most important thing in my opinion is to to make a good-faith effort to get an accurate ruling (steps 1 & 2) but barring that just make a quick ruling that won't slow the game down and move on with the fight. Keeping the game moving along is an important role of the DM, and no player enjoys sitting there while two other people debate the minutiae of a potential ruling. It's happened to me a few times where I've started a session with the players saying "Last session X happened, but it turns out Y should have happened. X will still stand as happenex, but in the future it will be handled as Y."
This varies hugely from DM to DM.
When you're at the table, the most important thing is to keep the game running. Keeping the game moving and everyone playing is more important than "getting the rule right" - so there's a number of ways to do this - and different tables have different feelings, but here are some options.
If your table cares heavily about "getting the rules correct" you can look it up. Some players place a lot of value on "getting the rules right", they want to know for certain what the rules of the game they're playing are, and it's important to them to have a consistent ruleset to play within. For tables with this mindset it can be worth taking a moment to look up the rule, so long as people are happy with the flow of play being interrupted. This is an exception to the general "Keep the game running" priority.
1a) 5th edition doesn't have hard and fast defined rules for *everything" (in this case you could use "improvised weapon" rules and treat the halfling as an improvised club)
1b) A lot of DMs are much more happy to "let something slide" if the player isn't gaining any significant mechanical advantage from doing it... So interesting note: in 5e, ghosts are only resistant to damage from non-magical weapons, not immune, so "swinging an improvised halfling" is less good as an attack than just using your weapons. Lower expected damage, lower chance to hit, might damage the halfling. Since the player isn't even getting an advantage from doing this, many DMs would be happy to just let it happen.
Assuming you want to keep the game going, here are some other options.
If the game doesn't have a rule, you can use a rule you know from another game. 5e often doesn't have strictly defined rules for niche situations, but 4e and 3e often did have rules for those things. I often use an older rule I remember if 5e doesn't have a rule for it.
Go with your gut. Just decide what you think should happen given your understanding of the game world and metaphysics, then go with that.
If your table likes it, use the "rule of cool" and just allow anything that is funny or interesting or cool.
If you're convinced a rule exists, but you can't remember it, a really good practice that I recommend is as follows:
\ a) In the moment, let the player do the thing in the fashion that's most favourable to them.
\ b) Let the table know, this is a one-time ruling for the current session, not a house rule that's permanently in play
\ c) Make a note of the situation in your DM notes
\ d) After the session, do the research to look up the rule, and take the time to figure out how you think it should work. (If you have mechanically minded players, they might be interested in helping with this kind of thing.)
\ e) Let the players know what the rule will be going forward. \
Fundamentally, what's most important here is NOT what method you use, but that your players know how you run the game and are happy with it. Any approach can work if the table likes that approach, and different approaches work better for different groups. There's no "right" or "wrong" answer other than "what works best for your group."
Best way is to do what the meme says. Make a snap judgement and roll with it. If a player objects, look it up post-game and make a modification. Don't retcon what happened, but give a bonus/penalty to a player in order to keep things in line. Or create a scenario in which the ruling would be correct and give the players a hint to discover why.
- Sorry what do you mean with your last sentence, "create a scenario in which the rule would be correct and give a hint"?
  Never played pen and paper, this question fascinates me.
Google it to see if someone else has a reasonable answer. If not then try to come up with one yourself based off of other rules in the game.
I'd go with the rules for an improvised weapon

I think you should be able to make unarmed attacks against ghosts. After all, you have a soul. There's a ghost fist inside your flesh fist. You should be able to hit them with that.

If you allow that magic platemail, any magic item should work. Save all your useless and cursed magic items to use in fights as improvised weapons. That also means there's no a reason people would be keeping cursed weapons for you to find in their loot.

Take Path of the Giant and swing your full size companions around for extra reach.

Honestly, I’d let him cook