Uploading audio and managing your discography is built for desktop, so the upload tools are hidden on mobile by default. If a phone is all you have, you can still do it. It takes two steps.
1. Switch your browser to desktop mode
This loads the full site so the upload tools appear.
iPhone (Safari): Tap the aA button on the left side of the address bar, then tap Request Desktop Website.
iPhone (Chrome): Tap the ••• menu, then Request Desktop Site.
Android (Chrome): Tap the ⋮ menu in the top corner, then check Desktop site.
The page will reload. Go to your dashboard and you'll see the upload and discography tools.
2. Make sure your audio file is on your phone
When you tap to upload, your phone opens its file browser. Your track needs to be saved somewhere that browser can see.
On iPhone: Save your WAV, MP3, or FLAC to the Files app first (iCloud Drive, On My iPhone, or a connected service like Dropbox or Google Drive). Audio inside the Apple Music app can't be uploaded. When the picker opens, choose Browse and find your file.
On Android: Your file browser can usually reach Downloads, Google Drive, and connected storage directly. Pick the folder where your track is saved.