Quick rundown of the Extraverse:
The Extraverse is a parallel universe to the FSNverse because of Extraverse spoilers.
The first installation of the Extraverse is Extra, and it has two routes.
Extra CCC is the second installation of the Extraverse. It is a parallel route to Fate/Extra's two routes.
Extra CCC Fox Tail is an alternate story to CCC, taken from the perspective of Tamamo-no-Mae.
Extella is a sequel to a parallel universe occurring during the time of Extra/CCC. Extella/Zero, the "prequel" to Extella, was written down in Nasu's diary. Extella also has multiple routes.
Extella Link takes place after one of Extella's routes. It's described as more of a side story than a sequel.
Extra Last Encore is also a parallel universe to both Extra and Extella Zero.
--
Someone correct me if I'm wrong in any of this. My Extraverse knowledge is admittedly fuzzy.