Every show, whether on a stage, on a screen, on a television as ‘extras’. Unless it is a one actor show or cartoon, all those people walking across the street, standing on train platform, shopping in a supermarket, are ‘Extras’.
Extras are actors who are hired to make up the background. They are not to draw attention, they are to be average. The eye is to cling to the star and the Extras pass by.
To be an extra is not impossible.
One has to register and pay their fee, and then get on some list somewhere. Find the agent who will be called to supply who is needed.
Everyone can be an extra.
Just finding the right agent is key.
By the way; everyone in that image is an Extra; hired and paid.