I’ve used sheets as a back to. I just did that, I also used a Cotten table cloth, I found at a thrift store, I’ve used a sheet for the front of a quilt too

I use flannel as batting a lot for smaller lap or throw quilts. I don't live in the cold north, so we don't need the heavier thicker quilts. On baby quilts, if I am using flannel as a backing, I like to use a piece of muslin for the batting just to pad the seam areas. While I do not usually prewash fabrics, whenever I am using flannel - wash everything.
Welcome to the world of quilting. Reminder - it is your quilt - make it however you want, the quilt police will not be coming by!!!!

I have made a quilt with out any batting/fabric and found it wrinkle very easily and does not have any body. I also used flannel as batting and found that the quilt was just as heavy as using batting and flannel can be way more expensive that batting. IMHO

I was thinking the same thing, that flannel can be pretty expensive compared to batting. But if its what you want to use there's nothing wrong with that

my only concern about not using batting is the wear. I often wonder if the seams from the the quilt top would wear through the back quicker without batting. I have see some older quilts that were worn through and needed a new back and I wondered if the batting would have prevented some of that wear. (but I don't know that - just my brain wondering)

I occasionally use fleece as a combination batting and backing.

I grew up in California and my mother always used old worn sheets instead of batting. A few years ago my son asked me to make him a new quilt as the one from my mother had worn so badly. He loved the weight and so I used 2 sheets inside his quilt. It was perfect for him and he uses it every day. When I was sewing his old quilt back together as best I could, I saw that she had used one old sheet and several muslin feed bags inside his quilt. She stitched the feed bags together to make a piece the size of the sheet.