I know this is probably easy to sew (a little mat), but Sewing Machines Parts and Sales in Los Angeles

I want to make a cage liner for my pets that looks like this. I have all my materials, but I don’t know how this Etsy crafter sewed this with that extra inner line of sewing. Did she maybe sew it all inside-out and then invert it through a small hole then sew that little inner border? I’d love some help as I don’t even know what terms to use in Google! Thank you SO much!