It’s Better to do a set routine for a while, so your abs get stronger and then once it starts to feel like a routine, it’s time to change it up and try something different. Then when that gets easy, go back to the routine. I have been doing the same routine for 20 years, off and on. It always works and I try different options once in a while, but I always go back. I’m in my 50’s and I still have a six-pack. That’s because the best way to get abs you can see is to stay lean. Once you have them, abs will always be easy to get back. The problem is “they go into hiding when you gain body-fat” Don’t forget the real core, the transverse abdominals! Plank away.
I’m not exactly sure what you’re asking. Do you want to train the abs in isolation, or in relation to the rest of the core musculature and body? And for what purpose? Remember, if you’re mindful about it (and I guess even if you’re not..), your abs are being worked and involved in many more movements than most people realize.