How do you put multiple videos into one screen?

How do you put multiple videos into one screen?

https://www.youtube.com/watch?v=6gEEVeOz0XA

How do I make a multi frame video?

https://www.youtube.com/watch?v=Bs0Is8qrU_U

How do I put two videos together side by side?

https://www.youtube.com/watch?v=B4izy3TMOj0

What is the shortcut to put two videos side by side?

https://www.youtube.com/watch?v=IrmmRh3uyOA