How to edit the default icon of Add Media button?

I am trying to replace the default Add Media icon in my WordPress site. I searched for the same on the web and got this:

How to replace default icon on "Add Media" button?

This seems to work for the OP, and ofcourse should’ve worked for others too. But, I am getting confused as the answer contains words like Child Theme and all. I could understand the first part of the answer, but I lost the track when the answer talked about this:

wp_enqueue_style('my-css', get_stylesheet_directory_uri().'/css/my-admin.css' 

From where did this part of the url came: '/css/my-admin.css. I searched in my database at the backend but couldn’t find this particular file anywhere. I know this might be a silly thing to ask, but this is it. Can someone help in this regard?