Google Keep? Not open source but it’s free and syncs across all platforms.
free and open source software
I keep messing with Memos. It uses Markdown which is stored in a SQL lite db, which I'm not super happy about, but I really like the UI so far. It's a web app only but there are 3rd party desktop and mobile apps. That said the progressive web app at least on Android is exceptional.
Sorry, should've mentioned you'd need to selfhost it. It is stupid easy to standup though. For "syncing" I just keep the wireguard connection to my server running all the time on my phone. Everything else is on the same network so it can be read from the server. Might be a bit too far of a deviation than what you asked for, but I'm leaving it. :)
You can sync for free on Obsidian using Syncthing. I use it to sync between my Android and Linux. Works perfectly fine
I have also tried notes using the CalDAV standard (see DAVx5 and Radicale). Used the opensource Tasks app on Android and Evolution on Linux. Needs a DAV server running. Complicated if you aren't into self hosting. Bad unintuitive clients
I've recently tried Anytype. Should be open source and works on the devices you described.
It's surprisingly polished, yet I've never heard anyone about it. So I wonder why that is. Not known or is it not FOSS? Would love to hear if anyone else has any experiences with it.