i expected the battery swap to be non-destructive….it destroyed EVERYTHING.
I believe I *may* have a full backup on external disk, but I haven't found usable instructions on how to (try to) restore it.
I can’t remember what all options I've tried but in short nothing worked and haven't found anything helpful online….but as a newb, searches may not have been optimal. I think I tried hooking up the disk then powering up the macbook (and the reverse) but if something is supposed to come up, it didn't.
this article should get you on the right track:
How to reinstall macOS - Apple Support
Note that early in that article, it also shows options to restore your Mac from backup, which may be preferable.
if you choose NOT to restore everything, On first run, your Mac will enter Startup Manager.
Startup Manager will suggest you create a new Account. You may be far better off if you point it to your backup drive instead. If you do, it will try to migrate your old setup, including settings if you wish.