I don't think I've ever heard the accusation about Trump and fascism fully explained.

The best I can figure, it's something like this: for many people fascism isn't the corporatism that Mussolini envisioned but rather fanatical racist nationalism. 
The tiny group of Americans who are KKK-style white nationalists probably support Trump.
Therefore, Trump is a fascist.

Something like that. 

 Amount many other things, if you just listen to Trump vilify “others” (esp non White people), and say they are running the country, and just replace those references with Jews, much sounds extremely similar to Hitler’s rhetoric. Hitler was a facist. 

 Hr wasn't a very good fascist because he didn't actually gas the fucking kikes  

 Fascism is a form of govt, not the description of the character of a politician.

Also, in my experience, most politicians vilify groups of ppl. Not justifying that - it's why I dislike the lot of them. But which groups of ppl has Trump said are running the country? I hadn't heard that