What format did old Macintoshes used?
You can’t get older than Apple File System with the first Mac.


https://en.wikipedia.org/wiki/Macintosh_File_System


The Plus ships with HFS so that’s the place to start if you just need that one model.

HFS Wikipedia article as well should cover all the classic Mac OS releases. So those two likely cover all the os from System 1 to System 7 so I would focus on what system you want to run on your Mac Plus (I’d start with System 6 if I were you) as opposed to what the filesystem might be. But maybe that’s all you need so start with MFS/HFS.