There's a lot going on in this Narb Makes video. First off, it's a reminiscence about the D&D game that Narb played ten years ago that introduced him to Warhammer 40,000. His DM placed their campaign inside of a 40K underhive (the bowels of a gigantic, towering hive city).
The party eventually made their way out of the hive and encountered space marines at the end of the campaign. To celebrate the birthday of that DM, Narb imagines that it's been ten years in the gameworld and the adventurers have all become space marines. So, as a birthday present, he makes a diorama featuring the 4 surviving characters as custom 3D-printed space marines.
The video teases the idea of using the 40K universe as a D&D setting. During the video, the Baron from Dungeon Masterpiece "calls in" to remind Knarb that there actually already is a 40K RPG (Dark Heresy) and that it's basically just Necromunda (the 40K underhive skirmish game) with added skills for character development. He suggests combining the Necromunda rules with some 5e D&D rules, adding a DM, and using the Necromunda underhive as a D&D setting.
In the course of the diorama build, there are some good gamecrafting tips, like creating a gouache wash. Oil paint washes are all the rage in game/dungeoncrafting these days. But they require the use of mineral spirits. A gouache wash is water-based, so much easier and safer to use.