Possible to temporarily hide BTT Touch Bar and show app default keyboard while holding a modifier key?

Hello! How can I show the app default keyboard while hiding the BTT custom keyboard by holding down a modifier key? Thank you!

I assume you mean app-default Touch Bar UI. If so, go to BetterTouchTool > Preferences from the menu bar. Then in Advanced Settings > General, there is an option to 'Disable Better TouchTool while holding shift/fn/ctrl/opt/cmd'.