Is it possible to create a "continuous layer" in Unity or would it be just a single giant texture ?

I am really a newbie, sorry if the question sounds stupid. Is a 2D layer always made by a grid of multiple tiles of textures or is it possible to make a single giant layer out of a material and, for example, clear perfect circles out of it changing its transparency programmatically ? In the case of a grid of multiple squared tiles I can't figure out how to make a perfectly circular hole in it.