I was wondering if there was any way of aligning circles in diagonal corners like in my first picture down below. Now I know there is a round corner effect and I also have a script that rounds only specific corners that I want, but I've been puzzled when I see people have an actual circle guide perfectly placed in the odd corner. I've seen it plenty of times on Dribbble snapshots.
I hope I understand what you want
Using the free tool from Astute Graphics called SubScribe
one of those great tools is to draw a circle ... ( and nothing except circle) between a given two tangents as follow.
First long press the tool
Circle by 2 or 3 points a flyout will appear as follow
Tangent Circle and select any two sides.