There are 17 states that require no ID. In my state they just cross your name off a list. Let's say I (hypothetically of course) knew my neighbor was not voting in the last presidential election. I walk in and give his name and address and vote. Then (again hypothetically) walk in again the next day and vote under my own name. There is no verification. You just need a name and address. And if it's the address of an apartment complex you can just give the address and see the names on the list as they're looking for it and vote a bunch of times as long as they don't recognize you.

You don't write your name or any identifying information on the ballot either. It's fire and forget.

If I had (hypothetically) done the above in the 2020 election I voted twice and did not get caught.