The claim by the left about 2016 is that there was foreign influence (and potential illegal collusion) in the election. There are fringe elements who claim that there was direct modification of votes, but the mainstream accusations are and were limited to things like advertisement targeting, funding, and potentially sharing of information between foreign actors and campaigns etc.

The mainstream claim by the right in 2020 is that there was widespread voter or election fraud (e.g. people voting multiple times, or people's votes being changed or vote counters not counting votes for a candidate).

There's a reasonable amount of evidence to support the assertions in 2016, including official government sources. There's reasonable amounts of evidence to conclude that many of the same tactics in 2016 were tried in 2020, including official government statements, but may have been less effective (due to awareness by the government, and policy changes by companies like Facebook and Google), and a lack of things like the DNC hack to make a big splash.

There's was no evidence of widespread voter or election fraud in 2016 or in 2020.