He totally knows he's lying. It's just justified in his narcissistic mind as what you do to get your way. If he's dementing, and I think he is, it's on top of that life-long sense of entitlement to get his way via whatever means necessary.
I think he does some intentional lying, and he says false shit believing that it is true, but most of what he says is bullshit. And I mean bullshit in the sense that he neither knows nor cares whether it's a true statement, he just knows that it's helpful to his goals in the moment, therefore he says it. When he talks about over 20M illegal immigrants entering this year, it's impossible for him to know a number on that since it obviously isn't tracked, but I'm sure that it doesn't matter what the actual number is at all. He just pulls out a big number to scare racist people into getting upset enough to come and vote for the guy who says that it needs to stop, which is him. He doesn't care that it's not an accurate number, and because he doesn't know the real number for sure then it's impossible to prove that he knows that it's not actually that number and therefore not a certain lie per se. He gets all of the benefit of lying without any of the attached accountability. He bullshits.