If you look at his patent,
https://patents.google.com/patent/US514169 it was basically just a pneumatic linear actuator. There wasn't anything fancy about it. It was mounted to the building, turned on, and the speed was adjusted manually until the resonance frequency was found and the building vibrated until it was broken.
These days you could do the same thing with an electric linear motor, or counter-rotating imbalanced motors and probably add a feedback system with an accelerometer to automatically find the resonant frequency faster than you would manually.
You also have to understand that a lot of the things Tesla said has been taken out of context and misinterpreted by idiots who don't understand how people born in the late 1800's spoke. For example: tesla's death ray shot a "particle beam" which Tesla made very clear were very tiny droplets of mercury, not subatomic particles. He also described a virtual "wall of force" using it, when a sentence or two back he was talking about "military force", saying that if you cross this line someone can shoot you with it, not some kind of sci-fi energy forcefield.
