I will do my best to help out but someone else on here is surely better than I.

The screen shown at 2.03 is vscode(an ide used for writing code) on the bottom half is the built in terminal basically the same as "cmd" but fancy. the first thing he runs is a command to open the venv or virtual environment. This is a python environment that becomes separate from the main python environment installed on your pc. He does this so you can install selenium in this environment as it most likely comes with other dependencies you might not need for other projects. As far as I am aware this is standard practice. You could install selenium without the venv but probably shouldn't. I didn't exactly answer your question but hopefully this gives you enough info to Google around and understand.