There are two types of early voting.

For in person voting, the local election board opens up fewer voting locations before the election. If approved, you can vote there and then instead. A lot of states eliminated any requirements to early voting after Covid.

For mail-in voting, the ballot gets mailed through USPS to voters. The ballots are usually supplied with an envelope with a way to verify identity, usually with signature. From there, the voter has the option to either mail the ballot back or directly deliver it to specific drop boxes.