How does Nmap detect OS?

Nmap detects the operating system (OS) by sending specially crafted packets to the target and analyzing the responses. Each operating system has unique characteristics in how it responds to network requests, creating a distinct "fingerprint." Nmap compares these fingerprints against its database of known OS signatures to identify the operating system running on the target machine. Factors such as the type of packets sent, the timing of responses, and the specific flags set in the packets are all considered during this process.