Where is the operating system of a Raspberry Pi stored?

The Raspberry Pi uses an SD card (or microSD card for newer models) as its main storage device. This means that the operating system and any other files are stored on the SD card, which can be easily inserted or removed from the Raspberry Pi. The SD card also allows the user to switch between different operating systems by using different cards or partitions. The Raspberry Pi does not have any internal storage, such as a hard disk drive or a solid state drive, nor does it use any external devices, such as an IDE bus, a firmware partition, or a GPIO module, to store the operating system. References:

Raspberry Pi OS – Raspberry Pi

Choosing Storage for Raspberry Pi - Kingston Technology

Beginner’s Guide: How To Install a New OS on Raspberry Pi