Best resource to understand backend/frontend interaction, web/mobile architecture etc
I am trying to learn about how various "parts" of a website/app interact and receive some practical guidance on creating a dynamic website/app that work together. I am looking for books, free online courses, tutorials, and hands-on lessons (etc.) to help gain this knowledge.
Additionally, some info on best practices would help. For example, explanations and advice on using AJAX and REST principles.
I tried to list a bunch of questions that are somewhat related. I know answers to some of them, but there are a lot of questions that I don't even know to ask. Some up-to-date resources would be highly preferred. Resources that explain how things work are preferred over very advanced technical documents. I appreciate any support.