Janet Warehouse is a location within the Neutral Zone that creates Good Janets.
Presumably, all Janets are made in the warehouse, but other Janets are more safeguarded; Michael, in Chapter 20, scorns the Good Place's trust in keeping their Janets in a "neutral pocket dimension", presumably referring to the Neutral Zone, so it is possible that the Janet Warehouse only produces Good Janets, and perhaps Neutral Janets as well.
Thus far, the only Janets seen in the Janet Warehouse are the Good Janets.
The Janet Warehouse is run by, presumably, "the Makers of Light, Darkness, and Everything", who created a Janet Manual built into each Janet and stored in their nostrils.
Janets are placed far enough away from each other that Architects selecting their Janets are able to select only one, but close enough that after one is removed another slides seamlessly into its place so as to increase convenience.
The Janet Warehouse is located in The Neutral Zone, "beneath the Shapeless Time-Void...[and]...right next to Accounting", according to Michael. Voids from individual Janets who originated in the Janet Warehouse appear to have seamless entrances into the Janet Warehouse, as the Janet Warehouse appears to be co-dimensionally linked to each Janet's Void.
However, each Janet's Void is (presumably) not directly connected, and therefore can be accessed via the Janet Warehouse. It is unclear where these backdoors exist, but they presumably exist on the spot where each Janet originally existed (before they were removed and the entire Janet line slid along, so that the backdoor entrance to a Janet's Void would always be moving with the entire infinite Janet production line).