Practicing improv is like practicing a language. You learn the words, then put them together into phrases and sentences, and then practice using them in conversation. In daily life, sometimes we say the same things over and over, and then sometimes we come up with new ways to express our thoughts and feelings.
It's the same with piano improv. You learn chords, voicings, bass patterns, scales, melodic concepts, rhythmic styles, and as you're learning them, you begin to put them together in new and fresh ways. And yes, sometimes we come up with new ways to express our thoughts and feelings through our music.
As for developing a style, and more complex arrangements, it's just a question of learning new techniques and then combining them on a tune. Not reinventing the wheel each time, although, yes, sometimes you play things you've never played before. The freshness lies in the context, just like in our daily interactions with people. Like right now 🙂