This article explains OEM Activation 3.0 (OA3). 

OEM Activation 3.0 (OA3) is a Microsoft licensing mechanism that allows OEMs to pre-activate Windows by embedding a unique, encrypted product key directly into a device’s BIOS or UEFI firmware at the factory. This eliminates the need for a physical product key sticker or manual entry during setup. When Windows is installed or reinstalled on the same hardware, it automatically detects the embedded key and activates the operating system without user intervention. OA3 simplifies deployment, reduces support issues, and ensures compliance with Microsoft’s licensing policies.

With this type of activation, physical product keys are not available, see this article for why.

For further information please visit Microsoft: OEM Activation 3.0 (OA3) System