“He does not think in a complex way at all.”
And believe it or not, that right there is the reason his base is so rabid in its support for him.
Put yourself in the shoes of a Trump supporter. Life has kicked you in the teeth repeatedly, and you're not bright enough or educated enough to recognize when it's been self-induced and when it's been caused by someone else. Chains of complex causality elude your understanding. The world seems complicated and mystifying, and all the Kids These Days^TM^ are talking in ways you don't understand about subjects that confuse you. You're not what anyone would call a "nuanced thinker," and maybe you legitimately don't even know what the word "nuance" means.
You view the world through a lens of black and white morality, and complex moral questions feel like confusing wordplay to you. You latch onto a form of religious faith that draws bold lines between Good and Evil. And you are instantly and immediately worshipful of anyone who tells you they have all the answers and can make the way you see morality into reality, and can solve the world's problems with simple solutions. It's even better if they tell you those problems aren't real. It's comforting in a confusing world.
You look at politicians like Kamala Harris and they say things that confuse you. You've heard of "word salad," so you mock her for it, when what's actually going on is that you can't comprehend what she says. Sure, you also frequently can't understand the sentences Donald Trump says, either, but he uses simple words, and if you focus, you can make the words mean something to you, and that's enough. He's like you, but he has the power to affect the confusing world and make it make some kind of sense. The ideas he presents to you - whether it's "build the wall" or "tax cuts" - are simple enough for you to grasp.
And that makes you feel smart. You can basically figure out what he wants, and it's the same things you want: Simple, sound-byte-sized solutions. Chanting them makes you feel good. So you'll follow him to the ends of the Earth.
Edit: I want to make something clear. I don't think these people are inherently stupid. I think they've been raised badly, they've been brainwashed by the "Fox News" infotainment channel, and they've been intentionally miseducated by nearly everyone in their lives.
Imagine you were raised specifically to not understand how computers work, and to believe that anything complicated anyone does on one is either a) gibberish, or b) magic. Then someone shows you a computer up-close for the first time. Not only are you not going to be able to comprehend the "desktop" metaphor, you'll probably believe all this talk about "windows" and closing or opening applications is just word salad. It won't mean anything to you.