How to handle different resolutions in a 2D sidescroller game?

I’m still learning, so bear with me.
I’ve got a simple project going on, which is a basic sidescroller.

On the above resolution, everything looks fine, but if I were to change the viewport size, the floor will either clip through the viewport, or it will be too high up on the screen.


So what techniques/tools do I have at my disposal to ensure that:

  1. The floor remains on the bottom of the screen, not under the viewport and not higher than it needs to be
  2. The game reasonably scales on different resolutions. e.g. if the user were to load this up on a 4K screen, the cat character would probably be smaller than a pinky.

This is just a project I’m using for learning, so it’s more about learning important concepts rather than pushing a game out into the wild.