... One guy did all this?
You're
done, AAA industry.
He used assets from the Unreal Engine asset store. The character models, animations, combat and coding/scripting, some vfx, cutscenes, story and voice acting are his. The rest was pre-made. Also, what you see is probably the majority of what he's made.
Asset creation is the most time-consuming part of game development, and AAA devs make almost everything from scratch.
Not knocking on the guy. He's done some seriously impressive work. Just contextualizing what you're seeing in this trailer.