I don't see refenece in this article or any others, but how did prosecutors get access to SBF's Signal messages?

Was it simply a court order that he unlock his phone (and agreed), or a codefendant who flipped to the prosecution and handed over the thread?

It's on Over/Shadow. If you don't know, the label is a bunch of old Moving Shadow artists and some really talented new heads. Definitely check out their other releases because there's good stuff in there.