You need a Flopper One to hack ATMs. Call Lester (Tools) from a Payphone blip to buy one.

Nearby machines show an ATM blip when you are close enough.

Press E at the ATM to start hacking. Finish the minigame. Fail or cancel and you walk away empty.

ATMs and your own cooldown need time between hacks. Cops nearby during a hack can raise the heat.

A clean hack siphons cash into your wallet. Rare jackpots pay a lot more. Expect wanted stars either way.