The person above you that originally asked the question used “illegals” as a noun. Some of the heat focused on you lis probably coming from people who aren’t paying attention to the usernames.
Constituents are the residents of an electoral district or the people represented by an elected official. Undocumented immigrants who live in AOC’s district are her constituents as much as anyone else.
As far as evidence goes, I think all you need is the second section of the 14th Amendment. Undocumented immigrants are counted in the census. I’ve read that if all of the undocumented immigrants suddenly vanished from California they would lose 2 to 4 seats (this could be wrong but it sounds believable so I didn’t dig deeper, feel free to fact check it) so there are literally members of Congress who only have seat because their constituency includes a relatively large population of undocumented immigrants.
I do but saying them gets you put on a list.