I dont personally create banner ads but i know people who work in the area and the ones i know use Google Web Designer program.
Dont know why its called Google Web Designer when its mostly a tool for creating ads from what i have seen but it does the work well if you have no coding experience.
Im stating if you have no coding experience because i have seen the code it generates and its messy, for a 160x600px banner ad it generates 3000+lines of code if unminified so its kind of hard to modify the banner manually using a code editor.