They don't have to directly. There are now lots of Leftist operated 501c(3) corporations that do the dirty work instead. These claim they're non-profit and non-partisan but aren't. What they do is register people to vote and get access to voter rolls. They then use those to ballot harvest. If an illegal got a ballot and wasn't going to vote, the 501c(3) people show up and ask them for their ballot. From there, it gets mailed in filled out as the ballot harvesters want.
I actually got a mistaken text message from a group called Mi Familia Vota in the 2020 election asking if they could come by and help me with my ballot. It was intended for someone else, but they got the number wrong. Yes, I turned it into the state for investigation of voter fraud but nothing came of that.
So, yes, illegals are voting. In what proportion? No idea. But they are voting either directly or indirectly, and it is hard to impossible to detect because they are legally, if improperly, registered due to those motor-voter laws and in blue states there is little if any attempt to check voter rolls against immigration records.