I use obsidian too! while it's not open source, there's so many community plugins and all your notes are stored on your computer in markdown, so you have complete control over your notes (you aren't locked in to using obsidian). I sync my notes with OneDrive, then I use the OneSync mobile app to get my notes synced onto my phone.

something similar to obsidian is logseq, it's open source but it's more geared towards bullet point notes. I used it a bit and it didn't make sense for me (I was mostly journaling, but it looks very promising if you take notes in a bullet point format!)