I was chatting to a coder about the video showing the machine vote rigging and this is how he explained it to me roughly.
You have to look closer at the video and note the greyed selections on the left everyone misses and doesn't comment on as well as the actual candidate names people are focussed on he thinks.

He obviously doesn't know the actual system or get to see the full picture but surmises the voter chooses the party (greyed selection on the left)  and then the candidate. If you look closely on the left you will see the voter incorrectly chooses a non republican party and then tries to choose Trump.
He doesn't select a democrat party either so in theory he shouldn't have been able to choose Harris either, if that is the way voting works over there. It is of course possible you are able to choose a party for say a senate vote and then a separate choice for President in which case it should have let him choose Trump. He doesn't know the system.

Either way the fact that it defaulted to Harris is the concern. He says it's possible to have everything else coded correctly, ie; select Republican party then select Trump or select Democrat then select Harris and it all works fine but effectively have a default to Harris for "exceptions" where dumb-arses get it wrong and keep pushing buttons. This can be worked into systems just by sheer code length of re-tries whilst appearing to be coded correctly when investigated as if you push the right buttons, you can select your candidate.
This would be a clever way to collect Democrat/Harris votes without anyone really being able to replicate the error. Repeated throughout the country, the Democrats would pick up thousands of accidental votes for themselves and the company could claim simple unplanned error, even when investigated properly. He explained this stuff can and is used throughout the coding industry to surreptitiously obtain desired results whilst appearing to be all above board.

It's complex and beyond discovery for all but really in depth, knowledgeable experts and even then able to be dismissed as accidental. If all are linked to a central coding system you can literally set it up and then alter it afterwards with no-way anyone can really track that alteration time of occurrence as well apparently.