I was solving the Extreme POD for 28th June 2021 and Greg mentions that there is a property of triangles where as long as they hit the same segment of arc , the angle is the same.

I didn’t really understand this property. What is the name of this property? I’ll search a bit on Khan Academy to understand it better.

Also, in the same video he mentions if a quadrilateral is inside a circle, the sum of the opposite angles will be 180. Is this true for any quadrilateral (rectangles, square, parallelogram)? If the quadrilateral is not within a circle, then the opposite angles don’t HAVE to add up to 180 (except for square and rectangle)?