It is a chain of loans
BRRRR stands for buy, rehab, rent, refinance, repeat. It gets talked about as a single strategy, but BRRRR method financing is really a chain: a short-term loan to buy and fix, a long-term loan to hold, and the capital that comes back out, used on the next property.
Each link is a different kind of loan with its own underwriting. The strategy works when each one hands off cleanly to the next, and it stalls wherever one does not.