/diy/ - Do It Yourself

Question here, maybe I understand it wrong but:
On a patchwork, the seams are hidden by doing them on the inside
On a quilt the stitches go through all three layers

So how do I do a patchwork quilt? Sew the patches together first, open the seams and then go exactly over the same line for the quilt part? Or should the ‘quilting’ be done in another direction to avoid having them just off

Also can I just use pillow filling (from cheap pillows I think it’s polyester) as batting?