My older machines all use SoundFlower which re-routes YouTube sound to Audacity Record. That’s obsolete and I understand this is the current software.
https://forum.audacityteam.org/t/problems-recording-samples-from-spotify-on-macos-solved/50779/3
If I had to guess at it, I would say Aggregate Device is following one of your Mac volume controls. Speaker Volume? That’s one of the problems of messing with multiple sound pathways on one computer.
This used to be a very serious problem on Windows machines. “How come I need to keep my speaker volume way up to get a good recording?”
Koz