Transactions on Cardano can be made up of many state changes. The transaction as a whole is atomic and a state change in the transaction will only be performed if all the changes can be performed.
For something like flash loans, as @Marek-Mahut points out, instead of having multiple transactions imperatively executed, each transaction just includes the before and after state. Flash loans are no longer needed in Cardano's model.
You could have a contract that allows you to redeem some ADA as long as you replace it with some interest in the same transaction--which is the equivalent of a flash-loan--but why would you ever do that? Plutus validators don't check that they're getting paid, they check that the state after redemption will be valid. The flash loan no longer serve the purpose of unlocking liquidity inside contracts. That comes for free with the EUTxO model.
tl;dr: All transactions on Cardano are made up of atomic state changes. This allows for what would be atomic sets of transactions on other blockchains.