Do you have a region/time period in mind? I know a bit about 18th and 19th century Western fashion, so I'll explain that, but if you are looking for something else I'd recommend expanding the questions details.

There's a lot of exchange both ways. You will see civilian clothing with distinctive military features. Women's riding habits are notorious for this. Reveres, epaulettes, hussar trimmings, color schemes, etc. can be picked up and translated into fashionable styles.

As for the other way around, it's general the civilian fashion that creates the base for military. So, as the 18th century goes on and the tails of the coats narrow, the cuffs diminish, and eventually the collars grow military uniforms will eventually mirror this. The standard uniform isn't restyled every year, so we can see that every once in a while they'll update, whereas civilian fashion for men is easily tracked from year to year. Interestingly enough, the written standards for uniforms are relatively vague. While the common soldier is issued a uniform, the tailors making these or the custom ones for officers go off of the same text. Often it will mention colors and where to place them, what weave of fabric, style and number of buttons, measurement and style of cuffs and collar, and perhaps a few other details like braid trim. The exact cut of the body is left to the tailor and will likely be whatever is fashionable at the time.