which comes first, resist or damage reduction from something like heavy armor master, goliaths stone endurance, or fighting style interception?

I think it’s the former then the latter but what do I know…

Here are guidelines explained in PHB & XGE about this:

Damage Resistance and Vulnerability: Resistance and then vulnerability are applied after all other modifiers to damage.

Resistance and Vulnerability: Here’s the order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability.

Even if multiple sources give you resistance to a type of damage you’re taking, you can apply resistance to it only once. The same is true of vulnerability.