Laptop Screen Brightness

What controls the brightness levels in a laptop? Is it the BIOS?

Is it possible to modify the brightness levels to have a custom brightness at each level?

From Brightness Controls on Integrated Display Panels:

In Windows operating systems, brightness controls are implemented in the operating system-supplied monitor driver, Monitor.sys. The monitor driver implements a Windows Management Instrumentation (WMI) interface to let applications—such as the operating system’s brightness slider—interact with the brightness level.

The monitor driver registers with the operating system device power policy so that brightness levels respond to changes in power policy. The monitor driver registers for Advanced Configuration and Power Interface (ACPI) notifications to process ACPI-based brightness shortcut keys.

An OEM can implement brightness control by using function shortcut keys such as Fn-F6 or Fn-F7.