As far as I understood the lore, neinzul are insects or insects-like (despite not looking so) that live in big space-ship hves and can also live/survive in space which kind of is needed considering they have a very small life span. Otherwise space flight would be never possible for them.
Zentih seemed always like Cyborgs to me. Because of their technological advancements they could fuse themself with powerful technology, allowing to create themself massive bodies. That may be wrong but their (initial) lore and their artworks indicated this.
Spire are kind of mysterious, they sem to be... space... rocks? That's what the journal pages said. Unfortunately the artworks go in different directions. In AI War they have a crystal-like appearance and that's what I've always imagined, they are living crystals. The SBR artworks let them appear like shells however, so i don't know anymore what to believe.