Foamily 4 Pack - 12" x 20" Hypoallergenic Lumbar Stuffer Pillow Inserts Sham Square Form Polyester, Standard/White

If my pillow covers are 20x20, should I get the same size pillow insert or 18x18 pillows?

I’m trying to understand why it need a 20x20 insert if my cover is 18x18?

Are these the actual pillow that fit inside 18" covers? Last time I bought something with this description all i received were covers.

Is there a zipper access to remove unwanted fill? Thanks