A Few Questions About the Resistance Icons Below Monster Life Bars

I believe this has something to do with the monster's resistances. However, I'm not quite familiar with its nuances.

Does NOT having a Resistance mean they have 0% Resistance or Damage Reduction to that Damage Type?

Do the Gold Resistance Icons mean they're immune/heavily resistant to that damage type? Does the Silver Icon mean they have a minor resistance to that Damage Type? Are there other colors of icons that display the positive resistances on monsters?

If the icon is red, they have negative resistance, right? Are there varying intensities of red icons that display how much negative resistance the monster has?

This picture is from PoE2, in PoE1 negative resistance icons are on the left.

Given that nothing has changed since PoE1, all your assumptions are correct. Silver icon means positive resistnance, Gold icon means high positive resistance (I guess around 40% plus), red is neagative (the only negative one). No icon for 0 resistance. There are no monsters in both games that are (full) immune to some type of damage. But in PoE2 more monsters tend to be well resistant to certain damage types.