You can absolutely do that with an iPhone. There are a few ways, the most important thing is to make sure you have the space on your phone. I have about 800-900 songs on my iPhone 16e, all bought through iTunes, then I also used to carry 100-200 songs from remix.kwed.org at the same time. Based on your post, I’ll assume that you have a collection of non-iTunes songs that you want to transfer to your iPhone. To do that you have two alternatives: Connect your phone to your computer, and use the iTunes software, first import your music collection into iTunes, then sync it to your phone. Get the VLC app on your iPhone, it has a webserver which can be accessed through the local network, and you can upload individual tracks directly to the VLC app on your phone, it is however slow and annoying. Both of these apps support carplay. I hope that even if you ultimately decide against using your iPhone in this way, it at least buys you some time to save, do research and get the actual MP3 playing solution you want.