iTunes only downloads from the Apple store. So I have to guess your question is not how to download but add it to iTunes?
Remember I may guess badly since these are 2 different issues. I will have to know where/how/details about your downloading.
But to add a downloaded MP3 file or files to iTunes is documented. Read from this search.
https://www.google.com/search?q=how+to+add+mp3+files+to+itunes
Every time I download music onto my pc, it automatically opens up the file with Groove Music. How do I open it up with itunes instead?

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