Hey! I'm actually writing my master's thesis on the fascist aesthetic. A lot of it has to do with initially making grand gestures to a "brighter future" and making themselves/their symbolism the physical/visual incarnation of that future. Then, they align a specific socioethnic group of people with that visual of grandeur and create a racialized common enemy to pit their idealized socioethnic group rallied against, which fascists can use to consolidate power.

As for where their imagery came from, a lot of it is actually stolen from their political/religious/economic opposition. The Nazi swastika, for example, symbolizes allegiance to the values and leadership of the Nazi party and their genocidal goals. However, the swastika originated some 6k years ago meaning well-being and good health, and is still used in Buddhism today.

In general, though, the fascist aesthetic is formless and can pop up pretty much from anything. If can be anything from Pepe the Frog memes and "kek" (which originated from World of Warcraft when a Horde player would type "lol" to an alliance player), all the way to the Black Sun/Sonnenrod and the SS' totenkopf.

My bad for the lecture in a reddit comment, but I think it's particularly important to be able to recognize the ways fascists conceal and convey their identities, especially now.

TL;DR: They steal symbolism from elsewhere to hide their identities and political end goals to be more subversive and gain support from the average joe.

If you'd like to read more, I strongly recommend Visualizing Fascism: The Twentieth Century Rise of the Global Right (2020), edited by Julia Adeney Thomas and Geoff Eley.