BTR AMP
MP3 is the format most large collections are built on: universally compatible, compact, and accumulated over decades of rips and downloads. Playing an MP3 on iPhone is easy. Managing fifty thousand of them is not.
Why the built-in Music app isn't enough
iOS plays MP3 natively, so the question is not playback — it is what happens at scale. The Music app slows down as libraries grow into the tens of thousands of tracks, its Artists/Albums/Songs navigation is fixed no matter how your collection is actually organized, and there is no way to edit a track's tags on the device — a typo in an album artist field means a trip back to your computer and a re-sync. Getting files in still routes through Finder or the Apple Devices app in the first place.
How BTR AMP plays MP3
BTR AMP is built for exactly this library. 50,000+ tracks stay fast and responsive, and every track shows its exact codec and bitrate. The on-device editor handles ID3 tags — titles, album artists, cover art, star ratings — so fixes happen where you notice them. Custom tabs let you build navigation around your collection (a genre, a decade, your live recordings), smart playlists maintain themselves by rules, and custom tags cover the categories no ID3 field does. Your organization carries into CarPlay, and playback covers the details the Music app doesn't: a 10-band EQ, ReplayGain volume normalization, and gapless or crossfade playback. See the full feature set.
Getting your MP3 files onto your iPhone
No iTunes, no syncing.
Send files over Wi-Fi from any browser on your computer (Connect → Wi-Fi Transfer), pull them from Google Drive, OneDrive, iCloud Drive, an SMB share, or WebDAV, use AirDrop, or play straight from a USB drive. The transfer guide walks through each method, and the importing guide covers the details.