I would have thought we’d had a pre-baked answer to this already.
You’re on headphones, check.
Do you like recording on-line content like YouTube sound? The settings to do that, can do this.
Audacity > Edit > Preferences > Recording: [X] Overdub. That should be the only thing selected.
Audacity Device Toolbar > Microphone Symbol (recording). That should be your single recording device such as USB input, Line-In or Built-In Microphone. It should not be the setting you use for recording YouTube videos, such as Stereo Mix or What-U-Hear. Those aren’t real devices. Those are “fake” false pathways inside the machine.
Some USB sound mixers can do this as a production feature.
Am I close?
Koz