Writing a query letter is a great skill to master regardless of how you plan to publish and here's why. They get you used to reducing your story to just a few sentences. Remove any spoilers and you've got yourself a back cover/product description.
But if you're trying to get an agent or publisher to seriously look at your query, there are few things you need to think about.
Now, The Query Itself
Your query letter should be clear, concise, and to the point.
Showcasing 80,000 words in a paragraph or two (no more than 300 words) is no easy feat. But you can do it. As a Story Grid Editor, I regularly reduce books to three sentences. One for the Beginning Hook, one for the middle build, and one for the ending pay off.
Having a clear understanding of each part of your story makes reducing your novel into a paragraph easy but more importantly, it's effective. I've read many a query where the author didn't understand their own story. How can you effectively tell anyone about the book if you don't understand it yourself? It's hard and time consuming. But it will make your writing, your understanding of story, and your chances of getting an agent or publisher much better.
It's a Numbers Game
Sending out queries can feel like shouting into the abyss. But the fact of the matter is, every "no" gets you that much closer to "yes". So don't give up.