How to put a downloaded mp3 recording into Apple Music?

How do I add an mp3 lecture file that I downloaded from another website (not iTunes Store) to my Apple Music library/Playlist on my iPhone. I have an Apple iPhone 15, and I have Sonoma iOS 14.0 on my 2018 macbook Pro 15".

Where is iTunes? Isn't that required?

I would like to be able to listen to the lectures on my iPhone while driving (via Apple Car Play).

Thank you.