Stash

Stash has two modes. They decide what a sync actually does.

Offline mode is the default and what most people want. Sync downloads each track as a FLAC file (or the closest lossless source available) and stores it on your phone. Once a track is on the disk, you can play it forever without a connection. It costs storage space but no recurring data.

Online mode is for people who don't want to compromise their storage space. Syncing creates a local index of your library: every track in every synced playlist is visible and playable, but nothing ends up on disk. Play streams from YouTube on demand. Almost no storage, but a connection is required to play.