Local transactions would be in Yen (also all vending machines such as train tickets - very important for getting around).
I would say you'd want to have at least a few hundred dollars worth of Yen when you arrive, and you could possibly get more when you get there, depending on how your bank works. Another option is to get a prepaid Visa card which you can then use to withdraw money in Yen from their ATMs.
Something I haven't seen suggested in other comments is to rent a mobile wifi dongle and unlimited internet. This was fantastic when I travelled there since I could get directions and translation in real time through my phone without having to deal with the hassle of roaming charges.
The process to pick it up was super easy - either at the airport or main train station (can't remember which), and returning it was as simple as putting it in the provided mail package and putting it in a mailbox.
Some other things - in my experience Tokyo at least is not super accessible for wheelchairs and the like. I recall that the first subway station I exited from had a two story stair climb with no alternative, and this was hardly a unique experience.
Also, Japan still very much runs on cash. I don't know what it's like for you, but where I am (outside US), we've basically transitioned to a cash-free economy for 90% of transactions so that was a big change for me.
At a guess, it's a correlation between the Empire having it's head in the sand about it's gradual collapse, and the enshittification of Reddit over the trust thermocline.
Edit: sorry, I thought you said you were familiar with the source. In Foundation there's a Galactic Empire which culturally and technologically stagnates to the point that the outer provinces break away, leading to the slow but inevitable collapse of the Empire. A breakaway faction (the Foundation) seeks to preserve the knowledge and technology of the Empire to reduce the duration of "barbarism" until a new Empire can form.
Rock and stone?