There’s no way for a host to “monitor” breakout rooms without going into them.
When I’m facilitating a large Zoom event, I work with the client in advance to educate a moderator for each room from the client organization. Larger companies (I’m solo!) often bring in multiple staff to assist.
There is also the menu option to “request help from host”. See this Zoom Support article for details:
https://support.zoom.us/hc/en-us/articles/206476313-Managing-Breakout-Rooms