When you mail in your ballot you sign it, and the signature is checked to verify it is yours. If you vote for your friend or neighbor, then you need to steal their ballot and forge their signature. You also need them to not notice that they didn't get a ballot and not report it missing/stolen. The rest of the questions apply equally to in-person voting, which are all figured out.

Voter fraud also almost never happens because the person committing the fraud is taking enormous risk for literally zero gain. People aren't stupid. Most people don't even vote anyways, why are they going to go to all this effort to commit fraud.