Fantasy settings rely on magic, wonders, and monsters to create extraordinary circumstances. It is the things that don't belong in the everyday world that don't just make it Fantastical, but give the PCs something to focus on. In Fantasy, either problems are caused by the magic and monsters, or can only be solved with them.
And in fantasy, you can draw upon pre-existing myths and folklore, as well as a rich tradition of pulp tropes to come up with the contents of your setting.
Modern role-playing settings rely on factions, contacts, and conspiracies to create something extraordinary for the setting. A well written conspiracy, a couple of secret societies, or a criminal organization, combined with some well done NPCs are what makes a modern setting seem like something special and worth pursuing. And again, it is these groups and NPCs that the PCs must focus on as the problem and the solution.
In a Science Fiction setting we have neither the familiarity of a modern or pseudohistorical medieval setting to work with from the beginning. And, while a good science fiction story might include conspiracies and criminal organizations, they require a lot of context to be made useful or understandable.
In science fiction, advanced technology, strange planets, and alien races create the extraordinary circumstances that drives the player characters: aliens, strange worlds, or tech that either create problems, or are the only means of solving them.
Because Science Fiction it doesn't have the context of the modern world or a historical period or mythological backdrop, that technology has to be custom designed by the game master.
I am going to cover the basics of the technology and how you can use themes to develop the technology in your setting below:
Likewise, if your story is set on a single alien planet, then the teleporter or starship that got them there isn't particularly important, unless the PCs are desperately trying to escape the planet or something went wrong with their transport, the details on the transport are not particularly important.
When something is necessary for the premise of the setting, but the PCs are not likely to interact with it, it should be considered backdrop technology, that will neither create nor solve problems unto itself.
Technology that either creates or solves a problem is the setting is focal technology. This technology, how it affects people, how they feel about it, and how it works are all critical to the campaign.
This technology does nor exist for its own sake, however. It has to serve the thesis or the themes of the story in some way. For example, if you are creating a setting where "the line between man and machine" is a major theme, then having AI, androids, and advanced cybernetics all can suit the theme and add something by being there. Teleportation technology wouldn't serve that theme particularly well, and might actually seem jarring or confusing. Your players might find themselves far more interested in what the implications of instant travel on things like international politics or crime might be.
When you are creating a game, asking yourself "How can I create technology that lets me explore this theme" is the best way to come up with new ideas that will fill up your setting quickly.
When you design technology along the themes, the setting feels more unified, which makes it easier for the PCs to understand, relate to, and immerse themselves in.
The difference between Background and Focal technology is often a matter of what you expect the PCs to do. For example, in Eternal Ocean, the adventures take place on the massive ocean that covers Rusalka. the starship Lorenzo that brings the PCs to the planet is not particularly important, and I don't bother giving it much character. On the other hand, a game where travelling from planet to planet and exploring the Cosmos is part of the campaign plan, starships are a lot more important than being just the thing the PCs are dropped from at the beginning of the campaign. The ship in an exploration campaign needs to be a character unto itself, with its own name, quirks, and details.
One of the dangers of designing a Science Fiction setting, or any TTRPG setting for that matter, is the temptation to throw everything that you think would be cool or fun to include from the kist of the latest and coolest ideas in popular scifi. these are thrrowaway technologies put into the campaign without considering how they might affect the way the campaign is going to feel or play.
Like my example of throwing a teleportation device into a cyberpunk story above, these can have a lot of unintended consequences for the feel of your game, the focus of your players, and the playstyle. (In a Cyberpunk game where crime and espionage are usually a focus, teleportation can really foul up how the game is played.)
As a general rule, the more unified your game is, the better a chance it has of creating a memorable, immersive experience that your players will agree on. The more broad and scattered it feels the more likely it is that players' attention will wander and the science fiction campaign will end before you are ready for it to.
I also would suggest that where possible, you want to keep the campaign world feeling familiar. If every aspect of life is different from our own, it is a lot harder to get into a PC's skin, than if things are different in a few, noticeable places. This is a trick that most of the successful science fiction writers of the past have done: make sure that the world is familiar where it is not intentionally strange.
An ultra high-tech world where we have no touchstone is difficult to enjoy. Moreover, it is harder to make it an internally convincing world because we have to make assumptions about how people would act differently with X, Y, and Z in their lives.