So this is the bit I’m really interested in? What are the unique elements that make Spelljammer special.
I've used it for 30 years or so in my homebrew, and I made some big changes. So my version is not like others. However, here are some of the ways Spelljamming originated elements are significant elements of my setting:
Neogi Slavers- The neogi are significant players in my campaign setting. If you're a neogi, you're a loyal resident of the Empire or you're hunted down and killed brutally. They originate on a different planet on the Prime Material Plane, but they use huge gates to take their Spelljamming vessels to the Astral, and from there to anyplace they want. They look for creatures to capture and enslave, either to servce in the armies of the Empire, or to sell to fund the Empire. They're known to have a massive fleet of ships that continues to grow and grow and grow ... waiting for some massive offensive that has yet to ben revealed.
The Spelljammer- The namesake massive city-sized sentient Spelljamming Mantaray shaped ship exists in my world, and it has been the location for two lengthy adventures over the years. In my setting, the city is a haunted ruin that tortures the Spelljammer and drives it to isolation.
Giff and the Arcane- The Arcane and the Giff are fairly tightly entwined in my setting with the Giff being the workers and the Arcane being their leaders. The Giff are free to serve of choose other life paths, but it is an honor in their society to serve, so few take the path of independence. The Giff are militaristic and loyal, with a passion for protecting the Arcane, which they revere in a way that many others revere Gods. The Arcane, are harder to find in my setting than originally described, and are fewer in numbers, but are the undisputed masters to Arcane magics. Most legendary magic items in my setting are a product of these creatures. It is unclear how many exist, but the number might be as small as 10.
Spelljamming Illithid- The majority of Illithid do not live on the Prime Material world. They live on Nautiloids that travel the planes in the service of their masters. In my setting, the Illithid are a creation of The Lady of Pain, the Queen of my version of Sigil which floats in the Astral Sea. She is more of a Cenobite (Hellraiser) style figure, and the Illithid on Nautiloids are loyal to her designs, but they serve without much interaction with Sigil or the Lady of Pain. They are a merciless police force for the Astral Sea around my Sigil, and travel far and wide to serve purposes that are often mysterious ... at first. The Lady of Pain is loyal to Cthulhu, and like all servants of Cthulhu, her methods seem chaotic, but result in surpring levels of order.
Spelljamming Trade- The major use of Spelljamming in my setting is trade. Massive Spelljamming vessels travel the Astral sea to navigate between gates that connect to places that have goods to offer. For example, dwarves mine my Elemental Plane of Earth and bring their wares to port(al) cities that connect to the Astral, where they are loaded onto these massive Spelljamming ships and then flown to my Sigil, where they are brought to gates that carry them to the major market cities (the Cities of Iron (Hell), Gold (Heavens) and Brass (Elemental)), or to major market cities on the Prime Material Plane. These ships often may a signel voyage in a year, but when they do they are heavily laden with incredible valuables. There are other smaller Spelljammers, but those are often less commercial and more pleasure based vessels. People enjoy sailing them even though spending coin to access portals is usually more economical.