https://download.tuxfamily.org/qownnotes/src/
https://github.com/pbek/QOwnNotes

QOwnNotes is the open source notepad with markdown support and todo list manager for GNU/Linux, macOS and Windows, that works together with Nextcloud Notes and ownCloud Notes.

You are able to write down your thoughts with QOwnNotes and edit or search for them later from your mobile device, like with Nextcloud Notes for Android or the Nextcloud / ownCloud web-service.

The notes are stored as plain text markdown files and are synced with Nextcloud's/ownCloud's file sync functionality. Of course other software, like Syncthing or Dropbox can be used too.

If you like the concept of having notes accessible in plain text files, like it is done in the Nextcloud / ownCloud notes apps to gain a maximum of freedom then QOwnNotes is for you.

Visit the project page here: QOwnNotes project page

To manage your todo lists in the web and on your mobile devices, you need to install the Tasks backend on Nextcloud or ownCloud.

To get more on-line features for your notes, like versioning and trash access, you might also want to install the QOwnNotesAPI app on your server.

To access your ownCloud notes from your mobile device you may want to get one of these:

For Android and OwnCloud: MyOwnNotes
For Android and Nextcloud: Notes
For iOS: CloudNotes
On Android you could also use any sync-tool like Synchronize Ultimate or FolderSync to sync your note files and use software like neutriNotes or Markor to edit your notes.

On iOS Notebooks may also work well (syncing notes via WebDAV).