Welcome Matthew.
Creating Animated Apps describes how to use Sprite collisions of your characters.
Here is a game that probably does some of what you want to do that you can use as an example (look upOh My Spikes – Create a game from scratch using App Inventor on https://imagnity.com/ in the Tutorials section of the site.
Also I’m making a 2D platformer (How do you make a Ball or Sprite 'jump'?) - #24 by SteveJG