Factory wipe + reinstall OS from graphene to stock.

Though I haven't done that in a while since I've repurposed or given my last few to SO.

For laptops that are disposed of, you can write 0s onto the full disk many times if you're that paranoid. Guessing that's possible with a (android) phone too but might be more complicated. Multiple factory wipes might do the same.

If you use a custom recovery like TWRP you can wipe data, cache etc. thoroughly enough imo

EDIT: A lot of this is probably overkill in 99%+ cases if, as someone else mentioned, the phone was encrypted as you should always ensure. Then a factory wipe would make that data unrecoverable. But sometimes, for some people, overshooting what the threat model realistically requires helps one sleep better at night, especially when these devices keep a lot of personal, financial/BTC information, etc.