I think you’re giving too much credence to someone whose argument was disingenuous. It was never about the security of elections, it was about people using mail-in ballots to not vote for him.
We may never know, every single time I see or hear this comment there is never any data/proof to back it up.
Remember: nothing was wrong with how we vote in 2016 and this suddenly became an issue in 2020.
Actual, intentional rampant fraud has not been documented.
You compare the signature with what's on the voter record. You check records to see if they passed away recently. Then you can compare addresses and any other personal details. This is more than enough information to strongly verify that the person who voted was who they say.
It's why fraud is rare and why the cases of attempted mass voting fraud are immediately identified (see: Leslie McCrae Dowless)