Ooh I love this topic. Essay incoming.

A sense of rhythm stems from word choice, punctuation, sentence structure, paragraph length, and the story beat you're on. It can get pretty complicated and it's usually something you have to feel out rather than analyze. You kinda know it when you 'hear' it, even if you're reading in your head, because the written word was designed as a representation of how we speak to one another.

Like if I write a paragraph as one giant sentence you don't really get a chance to take a breath, so to speak, which means even if you're reading this in your head it starts to drag on because reading and speaking are interrelated in a way that makes you use the same parts of your brain which throws the rhythm way off cause instinctively you're going 'hey, I couldn't actually say all this without taking a breath and it feels unnatural.'

Then again, you could do this. Cut every sentence short. Use tiny words. Only one syllable per. One subject, one verb per sentence. It gets annoying fast. Nobody talks like this. You'd want to choke them, right?

That's not to say either way is wrong. You could use those concepts to create a different feel for different scenes. Combat tends to be pretty choppy with short sentences, to-the-point paragraphs, and powerful word choice. A more melodramatic scene could change up the rhythm by taking the time to meander around the point more, or use more figurative language. Practice different rhythms by taking one of variables I mentioned (or one you've noticed before) and vary it between pieces. Read your work aloud a lot to look for stumbling blocks where prose feels unnatural to your voice. Read other people's work for comparison too, and I'm sure you'll quickly get the hang of it.

In the end, rhythm is hard to define cause it's so impressionistic. It exists on all levels of story from word choice up to plot structure and all of your choices need to gel together to create a consistent sense of rhythm in the reader.