Now, I’d love to hear your input on this, but as I understand it, there are two ways to round corners in Illustrator. Method one, apply an effect to the shape in question by clicking effect > style > round corners. Every corner of the object’s fill is rounded to the radius chosen in the ensuing dialogue while the object’s shape is left unchanged. Or, method two, use some combination of the Convert Anchor Tool and the Direct Selection Tool to manipulate or add anchor points until a rounded corner is achieved.
Many years ago, I had a job as a draftsman at structural engineering firm. It mostly consisted of deciphering some chicken scratches my boss made on a napkin during lunch and recreating them in AutoCAD. It was a pretty low level job but I learned a lot from it and really began to appreciate the precision AutoCAD offered. All software is math on the inside; AutoCAD celebrates this while Illustrator, I think, tries desperately to hide it. And if you try and use Illustrator like you use AutoCAD, you are in for some disappointment.
Aside from rounding corners, s.h. provides lots of other tools to help you in your ill-fated quest to use Illustrator like a CAD including sine curves, Archimedean spirals, and of course a script that instantaneously fills a layer with dancing people.