History of Vex Portal

In Shadowkeep, Ikora Rey said that Vex Chassis Components were being used to build a Vex portal behind her, and construction began with the latest weekly reset.

The components are common-rarity drops from Vex enemies on the Moon, especially during the Vex Offensive and Vex Invasion events. Their only known use is Ikora's weekly challenge "An Arm and a Leg", which rewards Powerful Gear after you collect a set number of them. The challenge unlocks after you have completed the Vex Offensive at least once.

The sources do not explain what the portal does once it is finished. Because the components kept dropping after the challenge was done, a later use was speculated when Shadowkeep launched.