To activate Windmills in Dying Light 2, you will need to climb to the top of Windmills using Parkour to turn them on via an electrical box.

However, before you can climb to the top of the Windmills, you will need to first level up your Stamina. Without leveling up Stamina, you will not be able to last long enough to climb all the way to the top of a Windmill in Dying Light 2.

That said, you can level up Stamina by collecting a total of three Inhibitors. When you collect three Inhibitors, you will be able to go to the skill menu shown below and increase your Stamina level. 

Doing so will allow you to last longer while climbing and using Parkour skills. Also, getting skills such as Firm Grip, followed by Fast Climb, and then Ledge Jump will help you climb Windmills.

That said, once you are prepared to climb Windmills, you will be able to make your way to the top with ease. When you reach the top, search for an electrical box as shown below. 

Interact with this Electrical box to turn the Windmill on, after which you will be able to unlock Faction Structures for the faction you sided with. 

For more on Dying Light 2, stay tuned for more coverage in the upcoming week and months after release.