I still give most mixes and masters a test listen in my car. Three hundred years ago I did this by burning CDs, but now I use my iPhone. Over time this has become much more of a pain in the ass. I posted mixes on a web server and for years listened to them directly in my phone's browser, but downloading MP3s or WAVs over the air is slow and seems to have gotten flakier through the years. I could load mixes onto my phone, but my studio computer isn't set up for that, and loading music onto an iPhone is clumsy and screwy in my opinion - probably because Apple wants us all to pay for their streaming service. I could use Dropbox, but I don't think their player is that great.
I knew a better mousetrap must exist. After auditioning a bunch of apps, I settled on Readdle's Documents. Documents is like having an exposed file system and Finder on your phone. It also has a built-in web browser. Plus, just like on a desktop computer's browser, clicking MP3 or WAV, or ZIP links will download files to Documents' filesystem. Within Documents you can rename and delete files, create folders, extract ZIP files, and so on. And importantly, Documents has a decent media player that behaves like a proper iPhone audio player: just like iTunes or Spotify, all of the phone's usual media controls work. You can switch apps, turn off the screen, and control playback from the lock screen. The Previous/Next controls changes tracks within a folder and feels like you're just playing music on your phone. Most of the other document apps I tried didn't include this crucial feature.
So, now when I upload a ZIP file of mixes for a client, or when a mastering engineer sends a WeTransfer or Dropbox link, I copy the URL then launch Documents to download and unZIP the file. Then the mixes are on my phone for as long as I want. No data connection required for listening - no fucking with ID3 tags to keep them organized. It's as friction-free as I can imagine. Plus, now my work-related audio lives in one place on my phone, separate from my Perfume Genius and ZZ Top MP3s.
One last thing: Documents has been in the app store forever and has millions of users. When I began using it, I emailed Readdle support with a few nerdy feature requests. One of those features I asked about appeared in the next update! No joke, this app has materially improved my engineering life. And for reasons I don't understand - it's free.