Plank pose: The body is supposed to be straight.
From plank pose one can exercise a lot of variations. They have all the purpose to make the body stronger. One can lift one arm or one leg or the right arm and the lift leg and so on. To stay in that position for one minute can be challenging as well.
It’s a great arm exercise when lowering slowly.
Important is to engage the entire body.
One can shift the weight of the body from one hand to the other.
I like that it’s easy to integrate some variations of this position when doing the sun salutations.
The core and the back must be seen together. Shalabhasana gives the opportunity to work on back strength.
All core strength exercises are preparations for the jumping back and forward vinyasa.
My experience is that it’s better to exercise a few core strength exercises that one can integrate in a practice than to aim for another strength practice. Repetition is a key here. Repeat the poses and movements that you like. After a while one can alter the exercises.