There are tons of Notes app available in the playstore and f-droid. I have use my fair share of them these are my best 5 recommendations. All of these are free to use and have to pay extra if you want specific features....
I have used both used GitHub for sync and Syncthing. Both work pretty flawless, but I prefer Syncthing (or rather Syncthing-fork).
As far as I remember, synching to mobile is not easy with the GitHub solution (I only used it to sync between two desktops), but Syncthing works great on both mobile and desktop.
Highly recommended note taking apps
There are tons of Notes app available in the playstore and f-droid. I have use my fair share of them these are my best 5 recommendations. All of these are free to use and have to pay extra if you want specific features....