Thank you HelenW for your answer, and i'm sorry, I guess that I didn't make it clear. Not sew a circle (fabric) to another piece of fabric. Quilting, the 3 layers together, top, backing and batting. Someone had shown Alex how to do the perfect quilted circle, and Alex commented that she wish that she had met this guest 6 months or so sooner. The guest had machine quilting,not had quilting.

Ladywingnut, I remember seeing a show a long time ago about pieced circles. After a little looking I narrowed it down to 206 Show Your Colors Bill Kerr/Weeks Ringle.
In that episode they show the typical quarter circle to a quarter square, but I found a you tube where they inset a circle by piecing it into a hole in the fabric. So
the term might be Inset Circles for finding various ways to do it. I attached a couple screenshots of the you tube I am talking about. HelenW

I remember an episode where the guest was showing Alex how to machine quilt perfect circles, (not applique circles) . For the life of me I can't find. Anyone out there know which episode I'm talking about?