The reasons are confusing to people who are used to thinking about things critically and systematically, informed by a knowledge of history and enlightened self-interest.
For example, a lot of Il Douche’s supporters are just rubes, desperate or otherwise, looking for a quick fix for their perceived problems offered by a snake oil salesmen. They’re ignorant of his history as America’s foremost public grifter going back to at least the 1980s when he was regularly mocked by Spy Magazine and Robert Crumb, and aren’t intellectually equipped or educated enough to see through a his scams if they were aware. Even now, with their names being sold as part of history’s biggest “sucker list”, they still don’t get it. They’re the kind of pathetic marks who will continually defend a grifter long after it’s become clear he fleeced them, too stubborn in their pride to admit they were gulled.
Then there are the “spite voters”, who are so angry and bitter about their own thwarted sense of entitlement (often based in race and religion) that they’ll vote against their own self-interest as long as the candidates or party promises to subtract from the comfort and happiness of those they consider to be “undeserving”: liberals and progressives, PoCs, immigrants, intellectuals, the “coastal elites”, etc.
Neither group thinks these things through, though, so wealthy members of the groups they hate end up laughing at them (all the way to the bank, thanks to tax breaks for the top 1%) while they themselves suffer the worst effects of brain-dead protectionist policies.
I find that those are the two main reasons (sometimes both concurrently) why anyone voted for a cartoonish con man that you or I could see coming a mile away.
Besides that, there are the trollies @anon61221983 describes above and the alt-right bigots and misogynists. And then there are the establishment conservatives who think they can “control” right-wing populists and use them for their own purposes; as a German you know how that one plays out in the end.
I consider that more of a throughline, where many of his voters (especially in the Know-Nothing 27%) continue to long for a “strict father” leader or government to somehow make everyone as much “rugged individualists” as they (usually erroneously) believe they are. It’s a weird view where they believe that “my dad was a tyrant, but he made me into the self-reliant person I have today. All those liberals and PoC need some of that tough love too to make the country great again.”