Kett planet

The Kett main base is on Voeld according to the Angara, you can't even miss the reference, they tell you when you are asked to gain Efvra's trust and that's a main mission. We just never see the main base, outside the "temple". The Angara also tells you that the Kett mine for resources on that planet and there are drills all around the map. 

Kett also have mining ship, you can find a derelict Kett mining ship on the space map. They also drop supplies on other planets with orbital containers (you need to get someone out of cryo for Eos to learn that).

On top of that, going by the size of their big ships, they have plenty of space in there to have manufacturing in them. They don't need to manufacture new huge ships, the Angara do not have a fleet to attack them in space.