Great.
You need to use the paid sync option if you want full mobile funcitonality and also use the desktop apps. I guess icloud works if you are all apple only, but no E2E.
Another option on mobile is to use Ulysses to edit notes stored on whatever compatible sync service you are using, but you don't get the full Obsidian functionality (or E2E). If you mainly just type notes and don't do a lot of linking etc then it won't matter much.