In the age of streaming, especially music why would you want a music player that only plays local music? The simple answer is, even in 2021 we don’t have consistent mobile data networks. A recent example of this for me was when running in the 26th Taupo Great Lake Relay, on my leg there was no mobile signal. You still can’t rely on streaming yet.
An app like Doppler 2 is great for the above scenario. It can import locally to your phone all your music regardless the format with most importantly FLAC lossless music file support.
The importing options are huge with support for the files interface where you could import files from Dropbox, Google Drive etc, or in my case I prefer to connect via SMB to my shared drive on my Raspberry Pi. I can navigate to my music share (using the files app) and cherry pick music files from my collection while sitting on the couch. No computer needed.
If you do want to use the computer it allows direct copying via a web interface in your browser, point your phone at a QR code and you can then drag and drop files to your phone wether they be local on the computer (even copying from a shared drive is fast).
Well worth checking out with Car Play, great music file support and metadata editing on device (add the album art).