"Sorry about the "Multiple reps works endurance' thing. So what if you lift heavy loads, boost up your calorie intake and also boost up your cardio...?"
if you boost up your cardio [specially with some sort of HIIT] along with heavy heavy lifting, the two will just counteract each other leaving you back to step one. or worse; your body will end up using all of it's preferred sources of energy [which are glycogen and glucose]. When this happens your body then starts to breakdown it's tissue in order to burn the protein in the muscle as energy.
this is considered a Catabolic state and is a result of overtraining. (an 8/10 chance this happens, unless you are eating ALOT, have GREAT genetics or are on steroids)
this is a reason why you have people lifting the same 200 pounds they were lifting last year or have little to no gains whatsoever.
i tried to explain that as well as i could bruh.
hope that got the answer you were looking for