What you see here is a chatclient (irssi), vlc in another corner, and a terminal emulator running alsamixer.
The window manager is called I3. Which is a tiling window manager. First window you open is opened in full screen, any other window gets added next to it or underneath it.
BTW, for the eagle-eyes among us, the background that’s shown in the VLC video is a photo from the trainstation in Den Bosch.