Since someone brought up Obsidian

You want Joplin. It’s a markdown-based note-taking app, so it uses the same formatting as Discord. It’s locally installed so it works offline. It has a mobile version for iOS and android, but also has windows and linux apps. You can have multiple notebooks and multiple pages per notebook, so organization is easy as pie.

Did I mention that it uses markdown, so it exports into multiple common formats; that I’m aware of: JEX (their own) which is just a TAR of the text files and some other metadata, RAW which is the untarred version, HTML, and PDF. It also embeds images, audio, video and PDFs.

It’s also FOSS, and written in javascript using Electron, so it’s more-or-less easy to rewrite any part of it to suit your needs. It is also easy to work with plugins if need be, either from the community or writing your own.

It syncs across clients using some common cloud data stores: Dropbox, onedrive, NextCloud, WebDAV, s3, and their own self-hosted Joplin Server to name the ones I know. Make sure to encrypt. The local files (resources) that are linked in the notes sync across devices too. Web resources stay as links.

It also has a bajillion other features, but I’ll spare you.

No, I’m not getting paid for this comment (Joplin Team, hmu), I just really like this app.