WatchKit target file

Just a quick question: keep getting conflicting answers when I research this:
My app has a small section that would be good on a watch (real time game player stats). I’ve read I cannot develop a separate watch app and have to create a new swift file within my app that targets the watch so they’ll both have the same bundle id.
My question: When I resubmit my app, will the part that targets the watch automatically be listed as a watch app while the rest of it is ignored by the watch (and vice versa for the phone part)? Hope that is phrased clearly enough.