Spider-Man: Dissension was originally planned to set as a sequel film, but the creative director thought that the story should be told through episodic base to flesh out the story longer.
Julius is an original villain created by Nikolai Vadala that he see's as his own creation.
Mysterio was set to be a subway magician that got jealous by Spider-Man's fame leading him to his villain counterpart. Plans changed to flesh out his character in further projects.
The scene in Episode 2 "The Key" where we see Peter and Harry arguing than ending with a cliffhanger was filmed on 2 separate filming days
When Spider-Man got hit with the gun in Episode 2, Markos insisted to be really hit to make the scene look real, resulting in him having a black eye after filming.