Recently, California decided to allow illegals driver's licenses. Frankly, I'd rather have them licensed and insured, so I'm fine with that. Following that, the DMV began offering the option to register to vote, at the time you get your driver's license. This is one that raised some suspicions, because there was speculation that nobody was concerned with catching who was legal and who was illegal, when it came to registering. I'm not even going to pretend to know whether there are systems in place to catch this, or not. I'd rather believe there are systems.
I remember reading some opinion pieces, where some felt checking and enforcing would require exorbitant time and resources, so would be very difficult to catch those who registered illegally. However, an illegal's license is designated "AB-60", which is only considered valid ID for driving, nothing else. Yet, suspicions run high, but without any factual data to back them up.
So, yeah, Trump suspects, but he may be right, or he may be wrong. Other states have more lax voter registrations rules. Is it possible that some illegals voted? Probably some have, but how is it quantified?
Finally, whether or not it happened, I don't see what difference it makes to him. He won the presidency.
I guess, maybe, his ego won't let that rest, because he's all about winning.