I know some people use [[orgzly]] on mobile. and some [[obsidian]]. I’d love to do a proper survey of alternatives and be able to recommend a good solution that is also open source.
I use [[orgzly]] to capture my fleeting notes. It’s a great app. Available on FDroid. It’s uses org-mode for its backend file structure but you don’t actually need to use org to use orgzly itself.