Do you have an over active thyroid?
If not then your metabolism is pretty much the same as anyone else your height weight and muscle mass.
There are no such things as super fast metabolisms, some people eat less and move more than others, some move less and eat more, its that what makes up body composition.
If Zach Khan can gain on 4000 calories max then anyone can.
If you are eating that much and not getting fat then you have a shit digestive system, your body isnt utilising what goes in and you shit it out before its used or stored as fat.
Im like that, I can shit out food as it went in, mushrooms are a favourite.
what have you eaten today?
Yes my thyroid is slightly over-active but that's not the only determining factor when it comes to energy output.
I do not agree with your statement that everyone's metabolism is the same, that's just so wrong.
Just because Zhack Khan can gain on 4000 kcal does not mean anyone can. I know a guy who eats like a snickers and a pizza per day so maybe 1500 kcal on average per day with about 50g of protein per day at most and he looks great while I could never gain muscle on 1500 kcal.
It could very well be that my digestive system is not very efficient, I've always wondered about this too, to be honest. My shit looks pretty well digested though, I can tell you that

What I ate today ? I did not keep an exact log for today since it's a holiday and I'm just being lazy the whole day eating junk food... on days like this I often reach way more than 5000 kcal but many people doubt me when I tell them what I eat in a single day until they actually see it with their own eyes

They think there is no way that I can stay ripped all year round while shoveling down ice cream, cakes, junk food, chocolate... etc. in huge amounts.