You might have this amazing story that thrusts your fantastic characters into the journey of the century and teach the world the true meaning of life, but unless your world feels real, your readers will have a hard time investing themselves into your story.
There is only one good way that I know of to make a fictional world feel real:
Ask the big questions.
Don't just make a list of whats, or jot down a bunch of single word answers for a fill-in-the-blank worksheet. These provide a good place to start, a way to get your brain moving, but simple facts with no history or uncertain future do not create the deep worlds that drive some of the best stories.
Ask "why," "how," and "what if things were different?"
Perhaps you know how your government works. Why does it function the way it does? What would it take to crumble?
Perhaps you know the history of your people. What if they believe something else entirely wrong?
Perhaps you know each of your character's religions. How would each one respond if their beliefs were challenged?
Do you get the idea?
Real life doesn't hinge on a fill-in-the-blank list of everyone's occupation, family, and personality types, so why should you force your story to do the same.
Not only will your world feel authentic and probable, your story will also become easier to write. You'll start to see your characters interact with your world, not just live in it. You'll see layers in your worldbuilding develop and deepen the setting and mood of your entire story. You'll start to be transported to that world, not just viewing the world from a helicopter.
Sometimes the single word answer doesn't create the world you desire.
Stretch your brain.
Ask bigger questions.