Insert code before the_content()

Im looking for some advice on where I can find the template for the STANDARD post. I’ve checked everywhere trying to insert <!– TEST –> into all the files to see if it shows up, but to no avail.

I want to insert my plugin code to use [featured-img] function in every standard post as seen here: https://omggim.me/?p=342

Right now, Im just using it in the content, inserting the shortcode and then a BR. I want to hard code it into the template so I dont need to do it every time.

Can someone direct me to the correct php file I have to edit? I’ve gone through all them that made sense.

Thanks,
Jason