"Reduction to practice" is the requirement. You have to either build the thing or describe it in enough detail that proves that you have built the thing.

If that's the case how do these patent troll companies sit on thousands of patents? Or they likely milk it to the last day of that time window before it expires I guess.

Patent trolls buy them from people who reduced them to practice and then went out of business. Their inventions being copied is often part of why those companies went out of business.

Without patent trolls, there would be no reason for startups to get patents - you are just going to go bankrupt before the litigation finishes otherwise.