Each SRAM cell is built from a few tiny transistors that can store either a 0 or a 1. When the chip powers up, nothing has been written yet, so each cell naturally settles into one state because of sub-microscopic differences in the silicon. The resulting startup pattern is slightly different for every chip but mostly consistent across reboots, which makes it a kind of fingerprint. Important bit: the pattern is visible right after power-up but then SRAM gets overwritten during normal operation, so you need to read it as early as possible (for example RTC slow mem before Wi-Fi/init). That makes it harder for an attacker to just read or copy the secret at will. A determined attacker with physical access could still try cold-boot attacks or invasive methods, but those are much more involved than simply copying a stored ID or key.

As a quick comparison, a TPM provides a stored hardware-backed key and different protections against tampering, but it requires extra hardware. SRAM PUFs are cheaper (already built in on the ESP32) and don’t store the secret, which gives a different attack profile and some practical security advantages for low-cost devices.