Firstly, they collect nothing. They don’t even use analytics. https://www.skyjos.com/fileexplorer/support/privacy.html
Who is it? FE File Explorer.
It is a great iPhone file explorer, especially if you use Nextcloud and want to play music in the background.
It can connect via webdav and show all of your files. You can download them for offline use.
And unlike the Nextcloud app, if your screen shuts off or you switch apps, the music keeps playing. That seems pretty ideal.