Original iPods (and early iPhones) weren’t locked down as much. There were a number of utils that could manage your library. ml_ipod plugin for WinAmp comes to mind.

All my music gets copied into OS X's Apple Music, which still supports iPods. Other repliees are arguably better alternatives these days.

Rockbox.

on macOS you can still manage/sync iPods from the Finder (it was moved to there from iTunes when they killed iTunes).