I’m stuck now with sending the inputs’ values from the new liquid page to the backend (Express.js) using app proxies.
What does that mean? What are you stuck trying to do? Get what values? More details would be helpful. As an example of what I do for a pattern, I have a template in the shop that the merchant can edit. That same template is in my DB. When the Proxy gets a hit, typically the App is sent some data. An ID, whatever. Since I have this template, I fill in the Liquid that my App is responsible for. I used the ID to look something up, whatever. Now my template is a giant string of Liquid, with some Liquid tags filled in by me, and other I leave for Shopify. So I set the file type to be application/liquid and I send the string out the port back to Shopify. Shopify now renders the rest of the Liquid from the template, and WOW… what a cool thing. Right?
So what’s your problem? Try and explain it in some bullet points if you can. Simple is good.