"The Stem format provides the ability to freely interact with a track’s different musical elements. Each ‘stem’ can be controlled independently to create instant new mixes, mashups, instrumentals, acappellas, and more."
https://github.com/LMMS/lmms : free cross-platform alternative to commercial programs like FL Studio, which allow you to produce music with your computer
While it may not be ethical and/or legal depending on the source, you can rip and/or record from any music/audio source (YouTube, Spotify, Soundcloud, other streaming providers, etc)
"JACK (JACK Audio Connection Kit) refers to an API that provides a basic infrastructure for audio applications to communicate with each other and with audio hardware. Through JACK, users are enabled to build powerful systems for signal processing and music production."
Mixing
Native Instruments / Traktor
Serato
Etc
Harmonic Mixing
https://en.wikipedia.org/wiki/Harmonic_mixing
Learning
Tracks, loops, stems, etc
Concepts
Purchased
Free
Make your own
brew cask install lmms
Discover / Host / Etc
Ripping, recording, etc
Recording Mixes
brew cask install soundflower
+
and select 'Create Multi-Output Device'DJ City
Other Music Things
brew cask install melodics