The first thing. Any client you use has a set of relays configured to read from and write to.

Different clients with different relays will read and write to different places.

One thing that helps is (one day) setting up a personal relay that you put in all your clients so you at least have your own copy of all your own notes.

We hope (we being https://vaporware.network) to make it simple to self-host a personal proxy relay - meaning ONE relay you connect to which in turn connects to many others. All your clients would connect to JUST this one, which handles everything for you (better for device battery life and performance too). Not yet, though :)