Can someone tell me a free app or custom code that I should add and where to add so a Discount % badge shows up beside my product prices everywhere the price shows up
If you want add a discount badge to your product pages without paying for an app, or for a developer to write the custom code for you, I suggest you learn the basics of HTML and Liquid code (as well as possibly CSS) yourself.
Shopify has well-written documentation for Liquid. It’s a templating language that allows you to generate HTML based on the data on your site. You can probably get the hang of it within an hour. Here are the official docs: https://shopify.github.io/liquid/
As for HTML, while learning HTML in full will take you much longer, the good thing about Shopify is that most of the HTML is already written for you in the form of a theme. So, all you need to do is to learn and understand the structure of HTML, and you should be able to identify where in your theme’s product page code the prices are located. Then, you can add some markup to display the discount percentage badge.
I recommend the MDN guides for learning HTML basics; it’s the company that develops the Mozilla Firefox browser: https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_web/HTML_basics
Depending on how you want the badge to look, you might need to also learn the basics of CSS. A discount percentage badge is relatively simple, so you should be able to get by with just the background-color, color, and border-radius properties. MDN also has a CSS guide: https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_web/CSS_basics
That should be all you need to add a discount percentage badge to your theme for free. Best of luck.