A copy of the lawsuit and amici curiae brief can be found online. The gist is that there are thousands of violations in registration, voting, counting procedures, and can also be seen in light of data analyses submitted to various groups.
There's also an ongoing investigation in GA involving voting machines, with videos showing how they are not secure and presented in election hearings. The gist is that the machines tabulate ballots with no guards against overvoting, duplicates, and blank ballots except that they are put in adjudication. For the latter, instead of simply accepting or rejecting ballots, the software allows users to modify ballots, e.g., fill in blank ballots, change votes, overvote, etc.