Why are you talking about mail-in ballots in a thread about Dominion Voting Systems?
Mail-in ballots work the same way they have since 1864. You fill out a paper ballot and mail it back.
> Biden only won 16% of counties. Only a few key counties need to be targeted to flip a national election.
16% of counties - when more than half the US population lives in just 4.6% of the counties. Remember that many counties are nearly empty and have few voters. Due to the rural/urban political divide, sparsely populated rural counties typically vote for the GOP in recent elections while densely populated suburbs and cities lean more towards the Democrats.
Biden’s numbers in general were similar to Hillary’s in 2016, but he gained ground in the suburbs due to white educated voters fleeing from Trump after the last 4 years. There is no indication that a “few key counties” were targeted - the shift was seen in all 50 states. Compare Texas suburban county votes in 2016 to 2020.