Sentinel manufacturing facility

A Sentinel Constructor Factory is a facility that produces different types of sentinels, such as Sentinels, Sentinel Majors and Enforcers, or in case of Onyx, Onyx Sentinels. The appearances and sizes of these facilities very greatly depending on their location.

Halo 2
These facilities float high above the ground, in an attempt to keep them somewhat safe from the majority of Flood if there is ever another major outbreak amid the Installation.

During Halo 2, they first appear in the level "Sacred Icon" when the Arbiter reaches the inner abode of the Sentinel Wall.

There is an opening cleft in the structure that is similar to the opening where he first entered from the Phantom. In the distance, above the Library, there are two large floating objects. Once the Arbiter exits the wall, one of these objects is hit with a few Plasma Mortar shots (possibly fired from a Flood captured Wraith to slow down the Forerunner defenses) which then crashes down to the ground.

In the next level, you would be able to observe the same crashed structure being pounded by Wraiths and Scorpions and fight through it as Sentinels and Flood fight each other.

While the appearance inside the structure is badly damaged, some believe it to be a futuristic assembly line allowing the Monitor to have endless waves of Sentinels to control future Flood outbreaks.

Onyx
On the planet Onyx, there was a Sentinel Production Facility which constructed Onyx Sentinels every six seconds, quickly making a mass of Sentinels prepared to combat any known threat. It was later destroyed by the Spartan-II's and III's to halt the production of Sentinels on Onyx.

The facility was vastly different from the ones seen on Delta Halo, being a massive underground room under Onyx's nothern polar region. The facility was so large that the slight arc of the planet's curve could be seen in the distance. Thick black clouds drifted two-thirds of the way from the ceiling.

A gargantuan machine spewed a river of molten alloy into the air. This liquefied metal arced up and then cascaded into a hollow tower that pulsed with bioluminescent colors. From the bottom tumbled countless tiny parts winking with light. These parts were whisked away by ribbons of shimmering energy. From the opposite end streamed a never-ending procession of three-meter cylinders.

In the room, there was a pyramid-shaped structure described as being "five times the height of the Great Pyramid of Giza". The structure was composed of floating golden spheres that turned and glowed with Forerunner hieroglyphs etched upon their surfaces. Every six seconds a sphere from the apex of the pyramid ascended in a shaft of silver light. When the sphere emerged, three rods accompanied it, all parts spinning in null gravity, flexing, until the pieces settled into their places - a complete Onyx Sentinel.

Halo Wars
In Halo Wars, there are Sentinel Factories but they are grounded, fixed structures where players are able to construct Sentinels, Super Sentinels or Protector Sentinels.

Trivia

 * The Sentinel Constructor Factories are oddly only seen in Halo 2 and not in the other two games of the Halo Trilogy.