Computers have two basic sound channels. Play from the internet or a sound file to your speakers, and record from your microphone to a sound file or send to an internet task. That’s it. There’s no natural pathway between them and that’s what you need to record internet shows.
Something has to run the play system to get the content and then loop back the music or performance into the recording system.
Where the loopback captures the content counts. If it grabs the music from the speaker system which I believe is what Stereo Mix does, then the music will assume the volume of your speaker volume control. If you set up a recording and then turn the speaker down so it doesn’t bother you, you just killed the capture volume.
Some of the fancier loopback systems grab the content before the speaker volume control and you don’t get the low volume effect. There is also the legacy process of assigning playback to headphones instead of speakers. Leave the volume turned up and take the headphones off.
It’s true Macs have never had built-in tools to do this. We’ve always had to use special software packages and setups.
Desperation method has us sending sound out to a stereo interface such as a Behringer UCA-202, and then loop back the sound with a physical cable.
https://forum.audacityteam.org/t/how-does-one-record-audio-on-mac/58980/2
The UCA-202 has a place to plug headphones so you can hear what you’re doing.
Koz