How do I create a zone for a battle royale?

Hello, so I've been doing a battle royale game for my schools technology fair, and I would like to know how can I do this, i mean, that it shrinks randomly and every x seconds/minutes(my teacher didnt want to help me btw, maybe he didnt know as it seems like something pretty difficult) Thanks

Could you please decribe more about your idea?

Maybe you can draw on paper is ok.

First of all, sorry for HD xD, my phone didnt have battery and got what I had (my 10 yr old tablet), then... I would like to do it like as it was the Fortnite's or PUBG's safezone, that it generates randomly and on the second "zone" the circle shrinks to a random place inside the actual safezone after X seconds. Btw, is there any way to change the color of the map that is not colliding with the zone?