Any good exercises to gain muscles on the lower stomach?
I liked that you asked about gaining muscle on the lower [abdomen] stomach instead of asking about how to get ripped abs. As someone who has dealt with weak and compromised pelvic floor muscles and hip flexors in the past, I believe that building strength and mobility in the lower abdominal/pelvic area is vital for increasing one's functionality level.
So it's technically true that the abdominal wall is made of vertical, diagonal, and horizontal sheets of muscle that are not separated into "upper" and "lower" sections. However I think it's okay to imagine the abs as being broken up into regions as it helps the exerciser mentally focus on correctly controlling the abdominal wall during ab strengthening exercises.
I think that the lower abdominal wall is easy to neglect and not properly engage in many traditional ab exercises.