Foam is a Roam-like that is open source, easily modifiable and assembled on top of VSCode but leaves you with plain jane markdown. I use a vanilla markdown editor on my phone for quick notes, and then fire up VScode when I need tools.

It's less an "app" and more a set of recommended extensions that work well together. At the end of the day you have markdown files in a directory, however you want to manage and sync them (git, dropbox, rsync, whatever).

https://foambubble.github.io/foam

My personal setup also integrates heavily with my Zotero toolkit for tracking and storing papers and other materials.