There are apps like https://offpunk.net/ which are explicitly low tech (and even solar punk). But there are also apps, which just happen to be offline first (think PWA etc.). And things in between, like Syncthing. Some might be self hosted other maybe local or distributed applications.
What’s your favorite offline-first app or tool?
Keepassxc for storing/managing passwords. Also supports 2FA, which is great. I can just copy my password db over to whatever device I need it. Or sync it with syncthing