[pixelHD]

Adding to this, it has webddav support, so you can use gdrive/onedrive or even next cloud. I use my self hosted next cloud instance as the storage and sync.

And for mobile, i use this seemingly now-abandoned app called papership [ios][0]. It lets you signin to zotero and your webdav provider, so you have a sync of books/papers, along with your highlights, notes and whatnot. Thankfully, it seems like zotero are hiring ios devs, so a first party app might be on the horizon.

[0]: https://www.papershipapp.com/

edit: seems like papership recently started getting updates again, so its not abandoned anymore.