I've done a whole NTSB breakdown on that incident before, but here's what I hope is the short version:

He was using Pop!_OS. Pop!_OS' desktop environment was at the time kind of a fork of Gnome. I think now it outright is a fork of Gnome.

It just so happened that a version of the Steam .deb package went out with a buggy set of prerequisite data such that if it encountered a "weird" desktop environment it would declare itself incompatible with this which would make APT uninstall the entire GUI stack, right on down to Xorg. It wouldn't happen to distros using more mainstream desktops like Gnome or KDE or xfce, but it did effect weird things like Pop!_OS.

This bugged version was apparently the latest version published when the Pop!_OS install image Linus used was made, so that was the version in the apt cache on Linus' Pop!_OS machine.

In the time between the creation of that install media and the filming of the episode, the bug had been reported and an updated version pushed to the repo.

At no point during the install-first boot process, or while launching the Pop!_Shop did Pop!_OS update the apt cache.

Linus tried to install Steam via the Pop!_Shop's GUI. behind the scenes it saw the error about incompatibility with the desktop and threw a dialog box that said "Failed to install Steam." The system was not harmed or altered in any way and continued working correctly.

Instead of googling "popos failed to install steam" to see if there's a way to fix it, he instead threw a small bitch fit about how Linux doesn't work and you have to use the terminal for everything. He googled for "how to install steam with the terminal" or similar, and found the command "sudo apt install steam." Most guides online for installing things using APT tell you to run an update and probably an upgrade command first, I do not know if Linus found instructions that omitted that or if he skimmed too aggressively.

Running the command "sudo apt install steam" printed a lot of STDIO to the terminal including a large list of things it was preparing to uninstall, followed by a plaintext warning in bold text that read (paraphrasing) **WARNING! This operation is very likely to seriously damage your computer. You should not do this unless you REALLY know what you are doing. To continue, type "Yes do as I say."

It is my belief that Windows trained Linus to ignore such warnings, because Windows constantly throws errors about "this may harm your computer" basically every time it asks for administrator privileges. Linux does not do this; Linux usually accepts a 'y' or even just hitting the enter key with no input to mean "yes proceed," sometimes when it wants you to really stop and think it'll make you type the whole word "yes." Having to type that whole sentence feels almost like "update your last will and testament to continue." I think a lot of users learned it would do that from Linus' video.

He did so, the computer dutifully uninstalled the entire GUI stack and dropped him into a terminal.