Makes sense. If I use a standard Monero wallet like Cake Wallet to swap my XMR to BTC where is that swap getting the BTC from?
Yep and most all swap services you can't see the BTC until after the swap completes. The only exception would be on Bisq when doing XMR/BTC swaps with automatic settlement disabled. Then you can chain analysis the incoming BTC if you want from their deposit tx.