Can you make real money by playing games?

I have seen people all around the globe, earning money by streaming their gaming on twitch, etc.

I have a very decent gaming rig I have built, just a couple months ago... I have been playing a couple of popular games, Apex Legends, Team Fortress 2, CSGO, Rust, GMod...


How does such a system even work?
Is it legal?
How do you earn real money by playing games?
What is an average money earned in a single day?