The Democratic administration are attempting to use fears surrounding the coronavirus pandemic and the courts to push through changes to election law that they have been unable to achieve through state legislatures. A Biden victory based on late-arriving postal ballots should not be accepted by the American public, and it is illegitimate.
The US election administration is highly decentralised. Each individual state is responsible for setting its own election rules and local officials administering them. In the majority of US states, observers also keep tabs on poll workers. There are huge variations in voter ID requirements and voting machines across the US. Therefore widespread election rigging would be very difficult to coordinate.