Most cameras get indexed because Universal Plug and Play (UPnP) automatically opened a port on your router. Log into your router and disable UPnP. Then, shut off "Remote Access" or "Cloud Access" in the camera’s settings unless absolutely necessary.
The string is a specific Google Dork , a search query used by security researchers and enthusiasts to find indexed web pages that display live video feeds. What this search query does inurl multi html intitle webcam
Or more broadly to capture various formats: Most cameras get indexed because Universal Plug and