What is core?

In traditional yoga, most teachers dont say core. We are more likely to say uddiyana bandha.
What is your core? No hand wrenching here, just an honest question. If you talk to many powerlifters and strongmen, they feel they dont need to do accessory work on their core. When doing a heavy lift, they take a half inhale then hold their breath. Then they press against their weight belt and lock down the muscles of their torso. That is enough to keep their midsection strong. More pointedly, the primary muscle of interest is the transverse abdominus. It is natures weight belt. It wraps from the back around to the front. The muscles we think about most are what is visible, the six-pack. While beautiful, they are not as important.
There are many planes in space to consider when doing core work. It depends on which muscles you want to work. The key for me is variety. And, while hip flexors are important and should be worked, make sure you find exercises that focus less on hip flexors. For example, sit-ups are mostly hip flexors and less abs. One great exercise that I include uses an ab rolling wheel. It uses similar muscles to a plank, but on steroids. It doesnt take too many to wipe me out for days. While mostly working the rectum abdominus, I feel it hits a lot of the essentials without a lot of hip flexors.
See the attached video.