Under Abs there are three main major muscles namely External oblique,  Internal oblique, and the deepest layer which is Transverse abdominis. the first two are mostly in charge of rotating the spine while the Transverse  abdominis is the spinal STABILIZER which means it's one of the most important muscles around the spine.

Doing exercises such as PLANK, Side planks, stomach vacuum. In the mean time plank is the best and the intensity can be increased by time through the duration, and limps position variations.

Hope it helpe

If you don't feel it, that's because your abs are not braced. Doing any exercise without bracing the abs and packing the shoulders would involve different muscle group and is waste of time. For instance doing push ups while the shoulders are not packed and abs are not braced would make your back and shoulders get engaged instead of your chest.that's a common wrong movement I see time to time even among trainers.

hope it helped