Eventually Obsidian will have a mobile app and and and E2E encrypted sync service, but for now you have to get a little creative.
You can sync across devices by putting the root files into cloud storage like Google Drive.
To get notes on a mobile device you need a markdown editor that can sync to the cloud platform, I use iAwriter for this on Android.
Once everything is in the same location you can edit across devices and everything will sync effortlessly! You should be able to find a cloud based markdown editor that syncs to cloud storage as well though I haven't found the need to do so yet.