On Linux I use Zim Wiki. For android I use Joplin.

Zim is nice because the pages are just text files of known format and folder structure. You can actually go in manually in the file system and add/remove/modify/search/remove/rearrange if you need to though mostly the UI is just fine. I do not think there is an Android client. You could probably sync it through say Nextcloud because it is just files.

Joplin. This is more of a simple structure but there are clients for almost every platform and it can be synced securely though Nextcloud or another such service. So I use it on Android, and then also on Linux to access my Android notes.