I was curious to know if they built rooms or had dummies inside, or what they put in it to simulate weight.
There was no indication of any payload aboard Ship 24. Definitely no rooms or dummies. I haven't seen any documentation of a mass simulator, either, and much of the construction has been observed in detail, see e.g. this fandom.com wiki entry on Notable Differences between S24 and prior Starships.
In the past SpaceX has publicly acknowledged mass simulators / boilerplate payloads on test launches, so unless they release some statement contradicting this, I think "no mass simulator was observed" is the best answer we'll have.
Supplementary quote from Musk's post-flight Twitter Space (25m16s)
The goal of these early missions is just information. Like we don't have any payload or anything. It's let's try to learn as much as possible.