Science fantasy is a genre that most people will not know the name of but will have seen examples of.
Its the use of magic in space to put it simply. The best example of course is Star Wars, you could easily change the setting to a fantasy one with air ships. As a rule the themes it uses are more fantasy based then science fiction based. So fighting evil empires and such not trying to invent something.
Soft science fiction is where the science of a story is largely not based on what we know today. This can mean just having FTL, with everything else being grounded in reality. Or it could mean a dozen basic advances that defy what we understand of physics today.
You can tell good stories with either of these, however you should know going in which one you are going to do. It is easy to slip from soft science fiction to science fantasy and that can cause problems. If for no other reason then reader expectations you should make it clear.
You learn there is something more then high tech early on in A New Hope, if you only learn in the last parts of the movie that the force exists that could piss of people who started thinking it was a normal scifi.
When your writing science fantasy you can treat your technology like magic, it does what it does and that’s it. When your writing a science fiction you need to follow more conventions. For example in Star Wars the power needed to move the larger ships would be ridiculous, with that you would hardly need a death star.
If you are writing a soft science fiction, you shouldo think about things like how much power drives use and what that means for the rest of the setting.
That is really the difference, in a science fiction of any type you should think through the implications of the technology, if they have X, that means they can do Y, and that causes A, B and C problems. In fantasy you can’t extrapolate easily if at all.