Electoral Fraud does exist - and I think it's safe to say it exists to a certain extent in every election and in every country. The issue isn't so much about the fraud, but whether it was significant enough to change the outcome.
There is a good database on the Heritage website -
https://electionfraud.heritage.org that links to specific court rulings regarding US Election fraud. It's not insignificant... and before certain people jump up and down crying about the messengers - the data is authentic and documented.
I worked an election here in Oz at the behest of a group putting independents forward. We actually witnessed some anomalies with boxes of ballot papers being moved in personal cars.... it caused much bleating about Election interference.... However, I know that it made no impact on the result. The usual lines of people who would vote LibLNP (similar to Cons/Tories) or Labour walking up to vote for whomever would govern them harder... It's a tiny dent in a well-oiled machine.
Having said that - as a guy who has worked with data and patterns my entire life - the data shifts in some of those swing states in 2020 didn't seem quite organic. I mentioned it at the time. Whether it affected the outcome is another matter. Without cast iron evidence to the contrary, you have to accept it as it is.
I looked at the Heritage link you provided, without following ever single case, but a reasonable sampling. If you watched the 60 Minutes video that Tented up it, everyone will admit that some voter fraud does happen. But not in numbers significant enough to change the outcome, which is your measure. Same with what is up on the Heritage site. There are several cases of people not getting more than one extra vote, which is, frankly, stupid. (Some guy in NH voted in person in NH, and absentee in another state, in 2 elections. And he got caught! Some guy in Tennessee was a convicted felon, and registered illegally. And he got caught, too.) In a few, or at least 2, there is actual nefariousness of manufacturing fake names and ballots, but in the worst I read it was 70 ballots, which is not enough to tip an election. And, it was caught.
As to 2020, it WAS a pretty odd year, but Trump presented some 62 suits and most were dismissed or he lost. I think they won one. Also, Fox News had to pay some $1.6B US for falsely promoting the notion that the election was stolen. This should be the "cast-iron" evidence you need.
I appreciate your anecdote about working the polls in Australia. I've worked the polls here, too, 4-5 times. In NY, and I think in most places, you have police officers at the polls. They generally carry the ballots away at the end of the day, in police cars, but it's all properly sealed, in any case. As you say, even if you saw an anomaly, you believed it's generally a well-oiled machine, as it is here.
Point being, i don't know about everywhere else, but voter fraud in this country is basically not enough to say so. The Republicans fund-raise on the notion, so it suits them. And Trump continues to air his grievances over 2020, because he hates to lose.
What Trump did that was decidedly undemocratic, was refuse to concede the election, and he did not attend the inauguration of his predecessor. (Leaving of the insurrection at the Capitol, which is no small thing, either.)
