How did Trump develop such a devoted fanbase?

This is a complicated question. I think people that have responded thus far have failed to understand the difference between supporting a candidate and their policies and building a personality around him/her. I think that Trump originally appealed to conservatives who felt they were left behind by the Obama Administration and wanted to return to their roots (i.e "Make America Great Again") I am unsure of how Trump was able to create a facade of common ground between him and the average American, but clearly it worked for him and his admin. I think that he appeals to Republicans and Conservatives that want to conserve their country, their values, and what they are used to. His use of merch also provided identity and created a group for Americans who again, felt left behind by the former Obama Administration. 

Well, Chang, I can only speak for myself. There are a lot of things I don't like about Trump. Some of the things he has done in foreign policy is downright dangerous. But he has also done some good things. 

I went over to the Republican side because the media had been unfair, and some of the Democrats have been extremely nasty calling Trump supporters uneducated evil brainless redneck trailer trash. I don't like bullying and elitism. That's why I support Trump supporters. You also seem to be implying Trump supporters are not self-aware. That's simply your bias. 

He knows them personally probably and has direct ties with a lot of their higher ups probably who tell them what to do and they tell others. Then like all the white supremisit groups probably all know eachother and connect with each other. Then the higher up tell all the lower level pawns what to do and they listen. And everyone wants to fit in with each other for survival because of you don’t fit in you are as good as dead. It’s like that pretty much anywhere you go probably.

On the radio and television, stories about some illegal alien slaughtering a family while driving drunk came out, being criminals in the organized crime venue, rape, even of little girls, open borders that ALL the politicians who were wanting to be elected would promise to close (the Democrats too back then), the loss of funding to the forest service and other services, so many promises broken. Trump came along and did what was promised. The Democrats and the News media began to hate his candidacy, So it fell in line with the purposeful intent and contempt the Democrat have for the general public in America. (I could go on and on with more)

The Democrats have been in charge of Los Angeles for 20 years. The shores of southern California were so polluted by this trash and human feces in Los Angeles that the water in the ocean was toxic. Yet the Democrats want to make laws about the environment which oppresses the average American.

People that pretend to know what the "best interests" of others are normally those who believe that everyone is inept and dependent at they are. Or should be. 

" Fan Base " trivializes the importance of governmental representative selected to further an agenda and lead the country in a direction desired by 72,000,000 + voters .

Trump is not some fleeting K-Pop star .

I can't remember when people got so emotionally unhinged over losing an election as they did when Trump won in 2016. If Biden wants to help his supporters he should set up TDS support groups. It may be a waste of time and money, but their IS a cure. It seems to be akin to a spiritual awakening. See #walkaway.

Appeals to emotions through symbolism is a classic despot technique.

And Americans are primed their whole lives to be receptive to such propaganda. 

Because he’s the first president in 50 years to oppose the murder of children. Pretty simple.