New South Wales
Yeah, Youtube videos are not the best way to learn the game as many of them make mistakes.
The rules for spawning in buildings can be found on p20 of the rules. The main component of this is as follows:
Z:BP Rules p20 wrote:
Opening a closed building for the first time reveals all the
Zombies inside all rooms of the building (a single building
extends to all rooms connected by openings, sometimes
straddling several tiles). Indicate each Zone of the building,
one after the other in any order you choose, and draw a Zombie
card for each Zone. Place the corresponding number and
type of Zombies on the indicated Zone (see Zombies’ Phase
– Spawn, page 25).