How does "illegal immigration" impact this? You know undocumented workers cannot vote, right? Hell, you know documented, legal, residents also cannot vote? The only people who get to vote in US elections are US citizens.

Demographics shifts obviously help the modern DNC, for the same reason it helped the GOP a century ago: Conservatism is definitionally rooted in having minimal demographic variance (be it race, gender, religion, or whatever), so if you have two parties one of which says "only this group matters" and the other says "more than that one group matters", then increasing the demographic variation will always favor the latter, because for anyone not in the ideal conservative demographic will see that party as actively targeting them.

It's also important to be careful when saying "look at elections from the past", because a lot of laws were passed in many countries in the late 20th century to address a variety of biased laws that impacted the voting power of targeted blocks. In the US there was redlining which prevented specific demographics from voting in specific areas of the country, coupled with extensive gerrymandering this allows large scale disenfranchisement that is still being undone by the courts today, even though redlining itself became "illegal" in the 80s. There are a wide array of actions that occurred in the states you're talking about through the decades you referenced and onwards to now, that both made it possible for demographics to shift where previously people were functionally banned, and to reduce the disenfranchisement of demographics that were already present. None of that resulted in "illegal immigrants" impacting elections though, because again, non-citizens cannot vote in US elections (even though they pay taxes in the country that produced "no taxation without representation").

As an additional note, because of US government representation being determined by total population, and not "legal voters" the abuse of biased law enforcement is hits multiple times: first off victims of that bias lose representation, but then regions that get prisons get increased voting power. If you're in a region with 1000 people, but have a prison that has 5000 prisoners, then you get the congressional representation that comes from 6000 people, but only 1000 of them get to vote. Again, prisoners should be able to vote. Felons should be able to vote. Ex-cons should be allowed to vote. If there are enough of those groups to impact election outcomes, then it's a reasonable assumption that law enforcement is being used as a tool to restrict voting rights (Something that multiple people from the Nixon and Reagan administrations have explicitly stated was the reason for how the "war on drugs" was set up).