Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
Voicemeeter - Audio Processing and Audio Programming by V.Burel
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
VB-Audio Virtual Cable - Connect Audio Applications together with a Virtual Audio Device
Blackhole by ExistentialAudio - BlackHole is a modern macOS virtual audio driver that allows applications to pass audio to other applications with zero additional latency. - ExistentialAudio/BlackHole