A post double crochet is simply a double crochet that’s worked deeper. You don’t insert the hook into the stitch itself, but rather around the double crochet from the previous row below it and around it.
Either insert the hook from the front around the double crochet from the previous row, or from the back around it, and crochet a double crochet. I hope you can see it in the picture.
Bobbles are made by working several double crochets into a single stitch, which are then worked off together. The bobble is always crocheted in a wrong side row; it then forms, so to speak, on the opposite side.