A youtuber interviewed a trial lawyer [1] last year and I thought it was really good. I wasn't aware that the holding days could be extended for each crime that you "commit." There's an entire scheme to this:
3 days - initial detainment
10 days - judge determines whether you can be charged or not
10 days - possible extension.
Basically, don't come to Japan and do anything questionable and everything should be fine. In the unlikely event you're in the wrong place at the wrong time, you should know how to wield Japanese well to try to get yourself out of it.
On the other hand, I watched an interaction between a couple of foriegners and the police one day. They were drinking in the park one day (legal and common) and the police were called on them for unknown reasons. After the police arrived, they basically told the police to go away for an hour and refused to show their resident's card (both also legal). Backups and backups of backups were called and eventually they dispersed. I think this is a good example of how there is a lot of restraint with regards to police here. I believe in most other countries jail would have been imminent out of disrespect.
The thing about Japan is that once you're caught up in the system, it has no mercy.