There are four interrelated issues that all, to a greater or lesser extent, will decide the challenges.
The first is what I will call Election Administration issues. While present to some extent in all of the disputed states, this is most prominent in PA and WI.
The basic premise for these claims is that the Governor/AG/SOS/Election Board/etc. conducted the election in a way that violated the statute passed by the relevant state Legislature and this is a violation of the US Constitution (which vests sole authority over this topic in the hands of said legislatures).
These Election Administration claims are where the Trump team stands the best chance of a positive outcome. First, there is already a pending SCOTUS case (the PA case) that will have to answer the basic question and at least four votes for Trump exist (how Roberts and ACB will jump is unknown). If SCOTUS rules that state election law controls then PA will have hundreds of thousands of ballots tossed out and the ballots tossed massively favor Biden. It will likewise affect a great many ballots in WI and MI.
The basic thrust is that state law in all those states requires that Republican observers be allowed access to the ballot counting (and other parts of the process) and there is tons of evidence that the election administrators blocked them. Prove that fact in court and, combined with SCOTUS ruling that state law controls, EVERY ballot tabulated without the observers present is void and assumed fraudulent.
The second issue is Voter Fraud. This would be the dead people voting, the ballot harvesting of nursing homes, the alleged ballot stuffing in Detroit and Nevada, etc. The only two states where this is liable to have a real impact are Michigan and Nevada. In MI it is because of the Dominion whistleblower as her allegations call into question hundreds of thousands of votes, in Nevada it is because of the claims of ballots being filled out in a Biden-Harris van right outside the election office. Proving voter fraud to a level to get the courts to act is very difficult though. The real goal of these lawsuits is honestly to just get to discovery and make noise. Overturning the results would be nice but is unlikely to happen (excepting, again, maybe MI and NV).
Third we have election fraud. This is basically voter fraud taken to a systemic level and coordinated by election officials. MI is again the place this is most likely to be provable. It certainly occurred in Georgia as well but that wouldn't be proven without a results audit and the Governor and SoS went weak on having one of those.
The fourth is electronic fraud. This is manipulation of the electronic voting machines, tabulation systems, and poll books to advantage one candidate. This is basically the whole "Dominion" issue.
Of the four, electronic fraud is the most likely to alter the election outcome because if it can be proven then it will force detailed results audits of more than twenty states. Prove that the Dominion machines were switching Trump votes to Biden and Biden will not be President, Congress will refuse to certify him if nothing else.
Ignoning electronic fraud, the election administration issues are the next best path for Trump. Win the pending SCOTUS case and it becomes relatively easy for him to flip PA, WI, and MI as hundreds of thousands of ballots end up tossed. On the plain merits of the law, the Trump side is almost overwhelmingly in the right but at this level it is at least as much politics as it is judicial principals at play so what happens is * shrug *.
Voter fraud is a true occurrence but proving it to the extent needed under many years of court precedent to get election results overturned is essentially impossible.
Election fraud is tied up in the voter fraud and election administration issues.