As a longtime fan of the Tony Hawk's Pro Skater franchise, I have nothing but respect for people who are able to execute massive combos in the game. This video showing off an 18.8 million score is basically a miracle.
Building up a combo in the classic Pro Skater games is all about doing tricks and then increasing your multiplier. To do the former, you want to be maintaining tricks like grinds and manuals; to do the latter, you just want to be doing as many tricks as you can.
Figuring out the balance between holding tricks and doing more tricks is the work of getting big combos and video creator The Digital Assailant clearly knows how to manage that.
Despite the video being so short, it's almost a masterclass in how to play the later Pro Skater games. There's some great management of the grind and manual balance bars and you can see them getting harder and harder manipulate as the session goes on. There is even some great manipulation of the "off the board" timer that allows for tricks to be strung together.
It's worth noting that this is a recording of a gameplay session of THUG Pro, a modded version of Tony Hawk's Underground 2 for the PC and that's why it looks so much better than the Tony Hawking of your youth.
I've got nothing but respect for big skateboarding experiences and this is a pretty good one.