Historically, a is a type of activation exploit that mimics OEM (Original Equipment Manufacturer) pre-activation. Here’s how the old ones worked:

By midnight, the "free" OS had cost him his entire savings account. The loader was indeed portable—it had moved his entire digital life right out of his hands. The Reality Behind the Fiction

Activate Windows 8, 8.1, 10 and 11 Pro for Free - GitHub Gist

Microsoft publishes generic volume license keys for evaluation. These do not activate permanently but can extend a trial. Run as Administrator:

Downloading a portable loader executable file is one of the fastest ways to compromise your digital life. Because these tools must make deep, administrative changes to your operating system to bypass activation, they require you to disable your antivirus software. This leaves your system completely defenseless. 1. Malware and Ransomware Delivery

The "loader" aspect implies that the tool emulates a Key Management Service (KMS) server or hacks the Windows licensing system to trick Windows into thinking it has a valid volume license.

Since Windows 10 and 11 do not use the old "loader" method, they rely on these official features: Digital Entitlement