The system selects one of a set of competing miners to produce the next block via a VRF whose state is recorded to Bitcoin OP_RETURNs. There is at most one Stacks block per Bitcoin block, and the probability of winning is proportional to the relative amount of BTC burnt or transferred.
The system's design is well-documented via the SIP process. See https://github.com/stacksgov/sips (you'll want to read SIP-001 and SIP-007 in particular).
The system's design is well-documented via the SIP process. See https://github.com/stacksgov/sips (you'll want to read SIP-001 and SIP-007 in particular).