Trying to mimic the stamp in image number 2.
How do I create the white traingle that's inside of the pink triangle? (image number 2)
Use Appearance panel. You can add several strokes above each other and set them the needed width and colors. So, it will be white triangle with red-white-red strokes.
But what do you need this mimic for? We here don't support any out-of-law graphic actions.
With design in general. Terminology I'm decent but, still learning how to customize designs.