Centralized stablecoins like USDT and USDC aren't layer-one cryptocurrencies like Bitcoin or Ethereum. They're more like applications running on top of other blockchains, usually as smart contracts, and their issuers hold admin keys which can freeze, mint, burn, and otherwise redistribute funds at will. As far as timing goes, stablecoin issuers can't actually cancel or reverse a transaction that's already in a block that's been mined, but they can freeze the receiving address afterward and burn the funds there, then reissue them elsewhere, which has much the same effect.