The answer your looking for is they placed the sarcophagus over the reactor. It was a metal sarcophagus that was supposed to block radiation from leaking out of the reactor.
However, there was also other programs that occurred to secure the reactor/area to prevent further issues:
1. They sent liquidators to the area to lower radiation in the area (by unfortunately killing animals, putting contaminated items in certain spots, throwing graphite from the reactor on the roof back into it and more).
2. They evacuated the area (though arguably too late) and got most citizens out to prevent them from dying.
3. They sent minters to dig under the reactor and create a heat exchange, as to prevent the molten core from contaminating ground water.
4. They send a three-man team to release water valves in the reactor and prevent a steam explosion, which would’ve launched even more radiation into the environment.
