It’s for internal resources. You can really use whatever subdomain you want internally, but this decision would be to basically say to registrars, this TLD is reserved, we will never sell this TLD to anyone to use. That way you know that if you use it internally, there’s no way a whoopsie would happen where your DNS server finds a public record for this TLD.
I will provide a word of advice since you mentioned messiness. My original server was just one phyiscla host which I would install new stuff to. And then I started realizing that I would forget about stuff or that if I removed something later there may still be lingering related files or dependencies. Now I run all my apps in docker containers and use docker-compose for every single one. No more messiness or extra dependencies. If I try out an app and don’t like it, boom container deleted, end of story.
Extra benefit is that I have less to backup. I only need to backup the docker compose files themselves and whatever persistent volumes are mounted to each container.
I don’t have any answers to your questions, I would just like to mention that you can get complete images that do both of these things together. I use this one, but there apparently to be a bunch of different ones.
I encountered a similar issue with NFS a very long time ago. I had to set the option for each of my NFS exports to have a fsid and make sure the fsid is different between them.
So one folder has the option fsid=1
Second folder has fsid=2 and so on.
I hope this helps point you in the right direction.
.local is for mDNS addresses.