What is Pressure Vessel? It's kinda of like a simple version of Flatpak made for Steam games. Within the Linux Steam client, you can select "Steam Linux Runtime" under the right click -> properties menu of games at the bottom like so:
Want to see the dirty innards of more Valve code? Well you're in luck as they now have a lot of work involved in the Steam Runtime on GitLab including the Pressure Vessel container for Linux games.