Playing saved MP3 files on iPhone 11 pro

There is no iTunes on an iPhone.

There is the iTunes Store, used to buy content.

The Music app is used to play music.

If you want to play your mp3 in the music app, you will have to use a computer to put them in the music app.

https://support.apple.com/en-us/HT201253

Plug your phone into your computer, then open files as if it was a flash drive and look for the files, then transfer to your computer and put them where ever you want now, if you’re trying to play it from the files app then I can’t help you, this method is just to be able to listen to the mp3 files

Do you have a computer you can access? And if so what computer and what version is it running?