I want to transfer music from apple Music to iTunes on my iMac?

Can you clarify the question?

Are you talking about accessing an Apple Music subscription, using iTunes, on a computer that is running an old version of macOS? That works with High Sierra and iTunes 12.8.3 – and I'm guessing, also with Sierra and iTunes 12.8.2. If you don't have iTunes 12.8.2 yet, you can get it from here:

iTunes - Apple

Or are you talking about taking the Music Library from a Mac running macOS Catalina (or later), and translating it into an iTunes Library for a Mac running macOS Mojave (or earlier)? That might be trickier, at least if you wanted to take the whole library backwards in one step, instead of dragging and dropping songs to/from the Finder. (The Music app in Catalina and later might be storing the library in a different way than iTunes stored its library.)

if you use the same appleID then you have 100% access to the same music in itunes as you do on apple music app

no need to transfer or sync anything