itunes

Music in the iTunes Store needs to be paid for, either directly or through a subscription, unless otherwise noted. Music already on your computer can be put in the iTunes application and synced for free.

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To expand on what Niel has said, iTunes itself (the programme) is free. You can download iTunes to your computer from Download and use iTunes for Windows - Apple Support

My advice is that you download the version directly from Apple (listed on that linked page) rather than the Microsoft Store version (also mentioned on that page). The main difference between the two versions (as far as I'm concerned) is that the Microsoft Store version updates itself without telling you, whereas with the Apple version, you have to click a button.

You can add music already on your computer, obtained from online stores (including, but not limited to, Amazon and Bandcamp) or from your own CDs to your iTunes library and then to your phone (all for free).

If you have all the music you want, then you possibly don't need Apple Music (which is paid for) or any other streaming service. So the only thing you would need to pay for is additional music that you want to buy from online stores (as mentioned) and then add to your Library.