How do u get a “Booty”？

I used to have a nice round bum, and I never really hated my body. I struggle massively with my weight as I have IBs of sorts so my gut causes me.pain.and issues with digesting food. Over time my weight has dropped so low that Ive noticed my bum disappear

I want it back！！！ What exercises are best for building a booty please help a sister out ？any ideas for putting on weight also appreciated