This depends on what product you buy! If you have purchased the HD kit, the mesh points are hardware paired to that router. We have done this to improve the setup time and the user experience. That way it will be plug & play.
The standalone mesh points can be used in two ways:
1. Added to another AmpliFi device as a mesh.
2. Added to a 3rd Party router( non-AmpliFi device) in a mesh mode.
AmpliFi kits are not meant to be added to another kit. You can only add to the kit with individual pieces.