Any information on the Steem Proposal System (SPS)?

HF21 has been released with minor hiccups, which means the SPS is running and the @steem.dao is slowly collecting its currency distribution.

So the question arises, how does the Steem Proposal System work and what's the process to vote/interact with proposals?

I was under the assumption that @blocktrades had developed a user interface for the SPS that was going to be released with HF21. I'm also aware of the existence of https://steempeak.com/proposals and https://steemproposals.com/proposals, but neither of these interfaces has detailed information.

Does anyone else know how it works? I'm surprised there hasn't been more detailed information from the Steem core team.