EtherPK.

EtherPK is a note taking app where every note is an ordinary .md file sitting in a folder on your own machine. Local graphs need no account and cost nothing, and the editor holds two writing modes side by side: normal prose with headings, and an outliner for stacking blocks of thought. Nested wikilinks tie pages together and each one lists what points back at it. A paid plan runs an encrypted sync service that carries the same folders across your devices, and the desktop client is also published as source you can run from a Node bundle or a Docker image if you would rather keep the server out of it.

Most note apps hold your writing in a store you cannot read or move, and syncing means handing plain text to somebody else's server.