Synchronization between OSM and iOS

A nice app for OSM data with iOS is Pocket earth. Although there is no related desktop application, it is possible to export POIs / Pins etc in various ways, and Pocket earth has a web -based map accessible on desktop where you can see the bookmarked Pins. You might also download them as CSV list, an re-import it into desktop applications. It is described in the help on the web: https://pocketearth.com/fr/aide/ und import/export. Hope that helps. alfric

I also like Pocket Earth, but as already mentioned, there is no direct way of synchronising content from app to web due to the lacking desktop counterpart. What I like most about Pocket Earth is the ability to download whole map areas (or states or countries) for offline access.
There are many good apps which use OSM-Data for (mostly) beautiful rendered maps. Besides Pocket Earth, I can recommend MapOut (just export, no direct sync, but wonderful map and nice features). There are not many apps which offer a bi-directional sync for free; try GAIA GPS, or Viewranger (does not support synching POIs from app to web yet).
The apps I mentioned are mainly developed for hiking and other outdoor activities. Of course there are even more, like Komoot, Outdooractive, AllTrails, Footpath, Bergfex, Strava, FATMAP, myTracks or Bikemap. Just few of them offer the possibility to save and sync POIs.
I hope that helps as well!