Hi, I was just wondering why Rick used a circle collider instead of using a polygon collider which covers the shape of the sprite?
I’m not sure what video you are referencing, so I may be wrong when I say this but there may be multiple reasons, 1. It’s easier to make / add to the sprite as it takes little to no fiddling around, 2. Probably not this reason as I think you are referencing 2D, but in 3D a polygon collider is not a thing (however there is a mesh collider which covers the mesh / object exactly but that is not great performance wise if there are alot). I hope this answers your question!
Thanks for the answer but I guess it not covers my question.
I’m referencing to the Unity 2D course, lecture 103, at the Space Shooter Chapter.
In that lecture, Rick is creating a circle collider for the enemy spaceship and put it inside the shape so it wont effect outside the sprite. And that’s goes back to my question why circle instead of polygon. Hope I made it more clear to answer 
Just managed to have a look at the video, I presume Rick is using a circle collider to save a little bit of time and he most likely put it inside the sprite so the player doesn’t shoot close to the enemy and destroy it (which may make the game feel bugged even though it isn’t). Also yes, using a polygon collider would be best if you want your game to be a little more polished (until it comes to animated sprites where you probably don’t want to change the collider for every different animation frame). Hope this helps!