For anyone on iOS that is happy to use Apple’s own Music app as the player, copying your audio files to that device is generally pretty straight forward. Simply use the Music app on your Mac, or iTunes on PC, and highlight the tracks in your library you want to see. Hit Sync, and let the magic happen.
However, if you wish to use one of the more robust music players available from the App Store, you may prefer to bypass the above standard method, and directly copy over your files. There are several reasons to do this. One key benefit is to get FLAC, DSD, and other media types that are not natively played under Apple’s watchful eye onto your iOS device. The other benefit is to have a clear file management option storing your music under whatever folder and filename convention you prefer (like Music\Album Artist\Album\Disc-Track Title for example).
Known iOS Apps that play local files – BTR Amp, Ever Play, foobar2000, Neutron Player
(There are very likely more players that can be added to this list)
The Apple Way - The first method still involves the iTunes interface, but doesn’t use the regular syncing mode. Instead, you can use the File Sharing method that is also part of iTunes. You can drag folders and files directly to the player app’s document box here. There are some limitations, as there is no browser connected to this drop box. If you need to do any additional management, you can do this on your device using the Files app.
Windows (and MacOS prior to Catalina) - Use iTunes to share files between your computer and your iOS or iPadOS device - Apple Support (CA)
For Mac users on more recent versions of MacOS, you can manage access to your iOS device once connected to your MAC using Finder. For the music player on your device to have access to your songs, they will need to be in that app’s folder.
Mac (with Catalina or newer) - Use the Finder to share files between your Mac and your iPhone, iPad, iPod touch - Apple Support (CA)
Cutting Apple Out of the Picture - A second method is to use remote access from your iOS device to your main library. This eliminates the need for iTunes to do any of the syncing. For Windows users, this can be done using the Microsoft Remote Desktop app (or similar), and requires you to be able to turn on Remote Desktop on the computer that stores your music files. Microsoft Remote Desktop on the App Store (Note this option may not be available if you are using a “Home” edition of Windows)
With your device connected to your PC, you can then use the File Explorer to copy music files and folders from your main drive to a virtual network drive that will show up as something like "Documents on iPhone" or "Documents on iPad" in Explorer while Remote Desktop is active. Everything you copy to this folder will then show up in the Files app on that device, under the app folder “RD Client” icon. You may need to move or at least copy these folders/files to the separate folder for your music player in order for them to be found by that app. From that player app, you may have to go into the settings and have it refresh or sync the library if it doesn’t do this automatically.
Additional/Alternate Methods - Other ways to transfer without iTunes/Music are becoming available as some music player apps now support DLNA (such as Foobar and Neutron Player). While browsing the network accessed DLNA library, long-press the artist, album, or even just a song and select Download to have a copy transferred to your device for local play later on. This will automatically put the files in the app’s sandboxed folder location for you. BTR Amp has recently added the option to use SMB from within the app itself to access and sync files from a remote computer, manually or even via an Auto Sync job. Keep an eye on updates from your favorite iOS app to see if an update enables even more options in the future.
Personally, I have been using the non-Apple method with Remote Desktop for some time now, eliminating any need to use iTunes (Windows) or Music (Mac) to handle my library management. I have even removed the Music app from my iPhone12 altogether. I have been trying out the SMB method from BTR Amp recently with some good results as well.