I used to just keep a folder full of plain text files but recently (as in the last 2 weeks) moved to obsidian and the ability to add some simple markup (headings, code snippets) as well as search the content of the files easily has been a game changer.
Also it's just a thoughtfully designed, nice to use app (if you aren't an electron hater.. I daily drive VSCode, postman and Slack so am certainly not).
For syncing, you can either pay for Obsidian Sync ($10 per month, which is a bit pricy)
If you have a mac/iPhone combo, then iCloud sync is natively supported.
Other than that, Syncthing works well on Android/Windows/Mac, but I heard Syncthing on iOS is not that great.
Obsidian Git works well, and for getting stuff back and forth from a Git repo to iOS, Working Copy does the job.