Not an operating system, a framework
Despite the name, ROS (Robot Operating System) is not an OS in the way Linux or Windows are. It runs on top of an existing OS, usually Linux, and instead provides a set of tools, libraries, and conventions for building robot software out of small, independent programs that need to communicate with each other. Think of it less as a foundation you boot into and more as a common language and set of plumbing that lets different pieces of robot software talk without each one having to reinvent how to send data to the others.
This distinction matters because it explains why ROS became so dominant. Before ROS, robotics labs each built their own bespoke communication systems, which meant almost no code could be shared between projects, let alone between companies. ROS gave the field a shared standard, so a perception algorithm written for one robot could, with modest adaptation, be reused on a completely different one.