BellefitGirdle with Side Zipper Natural Birth Compression Garment, Postpartum Essentials

Looking at the behind photo of this product, I was wondering if it was just the model or does it not cover the full rear of everyone else who wears it

Am I supposed to wear this with or without underwear?

Can I wear this after a c-section?

Wondering what size to get small or medium?? I'm 5'1 and I already had my baby 5 days ago and I weight 142.