I'm very new to shopify app development and I don't quite know what I need for my idea.
I've already created smaller apps with react. But as I understand it, they are only for the admin page.
Question A: I can use react apps in my public shop for customers?
Question B: You can only use Themes with Liquid?
Question C: Are Themes only for the design or can i interact with my product and the checkout?
Answer A: Yes you can use React to create stores using the storefront API more details in https://shopify.dev/api/storefront
Answer B: Liquid is needed every time you need to get data from the server and render it in the store, I've seen stores that were just a landing page and in this case they wouldn't need liquid but whenever you need to deal with collections on shopify or products you will end up needing to use liquid unless you create a store using the storefront api as mentioned in answer A (my recommendation is not to worry so much about it liquid is very simple to learn to work with the themes in shopify you need to understand a little about theme architecture and how to use the input settings so that the retailer can change things like fonts, texts and images using customize)
Answer C I don't know if I understand your question correctly, but within the themes you can change both the design and functionality through the liquid, the checkout unfortunately can only be changed in stores with a Plus plan (which costs from $2000/month)
About scripts is another plus feature that allows you to create specific discount rules and is generally used to create progressive discounts or promotions with complex business rules
Hope this helps