finally found what i was looking for thankyou!!

The unique buildings are:
Ancient Cryo-Chamber (from Ancient Caretakers)
Phased Empyrean Shrine (Holy Guardians)
and
Master Archive (Keepers of Knowledge)

Officially, they aren't buildable because they are unique to the empires that built them. How is a 200-year-old empire supposed to build ancient cryo-chambers "containing the long-expired remains of billions of alien specimens frozen long ago"?
Unofficially, it's because I didn't see any techs associated with them, and thus I couldn't make them buildable.

What are the "unique buildings"?

No problem! If you've got any questions or issues feel free to comment or message. I can't exactly playtest this much since the techs come so late and with some of the highest research costs in the game, so it's great to have feedback

Thanks!

Just the buildings and ship components, I'm afraid. The ships are a whole other beast, but mods like Zenith of the Fallen Empires, and (I think) Ancient Cache of Technologies add in buildable ships/shipsets. For a shipset-only mod that is compatible with 4.0, there is the Ascendant shipset mod, but its not 1:1 with FE ships.

Will this mod add the fallen empire spacecraft? Or just buildings/ship components.