Guys, in my innate naivety, I assumed that he was of course not talking about simple 16 or 24bit (fixed-point) clipping because that's literally the first thing you should keep an eye on. But maybe I was wrong?

 

Gain Staging - lots of youtube videos on how to achieve proper gain staging on whatever DAW you are using

 

When your mastering engineer is sending your track back because it peaks at -0.1dB and he's to dumb to lower it himself, be happy and look for someone who's able to do at least the most simple things.
Additionally when you render in 32bit FP, the mainout can even have +120dB peak, problem solved. I hate YT idiots.