How do you sync?

Many mobile apps work offline to some extent and then sync their data when back online (think iOS Notes or Reminders). There are some solutions out there you can use, starting from fully managed Firebase Firestore or Realtime database, iCloud if iOS/mac only, CouchDB and friends, just carefully version your data and roll your own sync, Parse, I think there may be others.

I'd like to hear from people who have build an offline-first + sync app. How did you do it? Any thoughts on it you could share? Would you do that again?

On a non-technical level - is this something people expect from apps or can you get away with online-only or offline-only? I personally find it annoying when apps only work online, but as indiehackers, well, we're on a budget to ship.