I haven't gone too far into it yet, but the workflow seems to be select a tape for remixing -> download track(s) -> add/modify track(s) -> upload and update the tape to match what you had in your own multitrack.

So you do get the bits, putting it together in the browser is essentially so others can listen to it and do remixes of their own.

Yes this is exactly how it works.

tapes.fm wants to make the music making process more "social" and "effective" and not change the way how you actually make music.