To build huge abdominal muscles train them as if they were your legs, sure push ups/bench presses use the legs but expecting to grow huge legs without training them just because one does heavy benches is ridiculous.
Abdomen musculature is no different.
In response to:
Take a look at Eddie Hall (especially), Brian Shaw, Thor, or other strongmen. Their bellies look absolutely massive. The thing is, their bellies are almost entirely solid muscle and those guys carry far less body fat than most would expect. That said, sometimes they do carry a lot of extra body fat if they’re shooting for some sort of world record.
I have never seen any of them do ab wheel rollouts or anything like it. They build massive abs by squatting and deadlifting heavy weights. The military press with a proper Valsalva maneuver will also put a tremendous amount of isometric pressure on your abs if done correctly.
You can do 100,000 exercises for your abs, but heavy weights with a proper Valsalva maneuver will build abs like nothing else.
One could argue that the reason strongmen have huge abdominal muscles without training them is just because of insulin and steroids which are common among sponsored athletes. Plus recently Eddie Hall posted videos showcasing him struggling with doing a few sit ups and crunches. Meaning the strength of his abdominal muscles are not on par with his size, typical of drug users.
Not launching any accusations here, nothing wrong with using steroids, specially if allowed by the competitions and strongmen allows for use of drugs. Taking steroids is not a shame and it's fine as long as there is medical prescription and supervision. Steroids are like cigarettes or alcohol, dangerous and will kill you if abused but not a crime.
Also valsava manouvre is about air pressure, at most it trains the diaphram and the deep breathing muscles of the abdomen, it does not train the six pack or external obliques which work only as flexors of the spine. It's like saying overhead press builds huge dorsal muscles because you have to keep your shoulder girdle thigh or that deadlifts build your calf muscles because it keeps you from falling.
Holding air in your stomach,standing still or keeping a thight upper back has the same difficulty regardless if one does it with no weight at all or if they do it with 300 kilograms.
Why don't we see everyone with abdominals as big as Eddie's or other professional athletes? People do the valsave every time they defecate... After 40 years of defecation one should have enormous abdominals... But this isn't the case. The valsava does not build abs PERIOD
That's also the same reason people with good posture don't have giant lats and people have small calves regardless of how much they squat or deadlift. Stop inventing excuses for not training a muscle, just admit you can't be bothered training your abs/calves or whatever.