Can’t find my iTunes app for my subscription

You are sign in here .  .  .  with your Apple ID & Password (?).

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

Check (macOS & Windows OS) via iTunes:

Start Music -> Click on iTunes -> side bar click iTunes Store -> go to Account (top bar) -> View Account -> Sign in -> Apple ID Summary.

or -> (iPhone & iPad) Go to -> Settings -> tap your Profile. 

Hello ~ What happens when you click on the iTunes app on your iPhone?

~Katana-San~