When exogenous carbs are available, they are used preferentially, also at rest. Carbs need to go somewhere, and they only end up in fat storage after all other options are exhausted.

As intensity increases during exercise, the body starts releasing muscle glycogen. If previously the muscles were mostly using fat (because no exogenous carbs were available), now that endogenous carbs are ready, the muscles will use them instead. You cannot, however, reduce muscle glycogen use by supplementing extra carbs.