How do I transfer music files from my PC to my iPhone for playback using iTunes?

This is covered in the in iPhone User Guide. You should probably read it.

Apple - Support - Manuals

iTunes is what runs on a computer. Why do you want to transfer the files from the PC to the phone to then transfer them back to the PC for playback??? Do you mean playback using the Music app on the phone ("iTunes" on the phone is for connecting to the iTunes Store -- Apple is bad about calling different things by the same name).

iTunes 12 for Mac: Sync iPod, iPhone, or iPad - https://support.apple.com/kb/PH19458 - including manual syncing

https://discussions.apple.com/thread/7163823 - If you have signed up for Apple Music or iTunes Match you won't have the option to manually manage music/sync to a device in iTunes.

Yes, its possible, if you use iTunes:

Move content from iTunes on your computer to your iPhone, iPad, or iPod touch - Apple Support

Sync your iPhone, iPad, or iPod touch with iTunes using USB - Apple Support

Sync your iPhone, iPad, or iPod touch with iTunes using Wi-Fi - Apple Support

Sync media between your iPhone, iPad, or iPod touch and iTunes on your Mac or PC - Apple Support

Those should get you started. If you need to install iTunes first, then:

Get the latest version of iTunes - Apple Support

Yes it is. Just sync your music using iTunes on your computer.

Sync your iPhone, iPad, or iPod touch with iTunes using USB - Apple Support