how do I import mp3 files to music playing apps (whichever works)?

Where are these audio files located to begin with?

There is a solution, but it gets involved and it all depends where these mp3 files originate from to start.

Also, do you own any other mobile device or full computer?

The ONLY way I have found to play externally downloaded mp3 content is to download a third party app that is capable of importing ( sharing ) these mp3 audio files and playing them.

In the dowloads folder in the Files app, you will need to use the share icon/feature ( square icon with an upward facing arrow ) to import/share these files with another audio playing capable third party app.

Apps that I have found that work are third party paid notes apps with audio recording/playback capabilities/features.

The ones I have/use are

GoodNotes

Notability

Zoom Notes

Once you have paid/installed one of these apps to your iPad, In the downloads folder, in the Files app, you tap the square Share icon feature and look for one of these apps in either the horizontal or vertical app selection menu to share/copy/import these mp3 files into one of these apps and play these mp3 audio files within one of these third party notes app.

This has been THE ONLY way I could find to play externally downloaded audio files outside of the Apple Music app.

Best of Luck to You!

VLC is free and should be able to access your downloads from the Files App directly. Just copy any files you want to see in VLC into its folder in the Files App.

Link-> https://apps.apple.com/us/app/vlc-for-mobile/id650377962

Unfortunately to avoid piracy and conform to the record label's demands, Apple does not allow the Music App to import music from outside itself or iTunes on a computer. Its entirely a restriction imposed by the Record Labels, not Apple unilaterally.