Yes? In Australia every candidate can appoint scrutineers to watch votes being counted.

How do you trust all of them?

You only need to trust one of them, or trust that they would not all conspire to steal the election the same way. Not too hard, compared to a piece of software that has a much less reliable supply chain.

Yes?

Just livestream the whole process.

Or just stand there. Where I'm from, representatives from each party and members of the public will be present the whole day.

Mail in voting? Early voting last for up to 2 months. That doesn't scale to 100M votes

By "using paper ballots" I understood it to be in-person, voting by mail has a lot of issues indeed.

Usually in person voting is done through a machine. Where as vote by mail is a paper ballet

Only in the united states (where i am not from). Normally i think of paper ballots as the hand counted sort.

However as long as tge machine is just a scanner (still paper trail) much of the same argument spplies.

That sounds ridiculous but I'm sure it works this way in the US. If you can have the worst of both worlds why not make it so.