How do I learn web/API development? [on hold]

For most of my career, I’ve been developing utility functions for business software Read: implemented functions which leverage existing entry points provided by the core application using scripts.

Now, I am tasked with the implementation of standalone backend services providing and using REST APIs (e.g. using OAuthv2 Authentication/Authorization).

Suffice it to say, I am kind of overwhelmed with HTTP request standards. E.g. when is POST/GET/PUT/… the best choice, what belongs in the Header, what is best as a URL Parameter, or put everything in the BODY afterall?

Also, DNS and *-Proxy, Load Balancer, %insert web technology here% use cases are not obvious to me.

Sadly, I am not the kind of guy who likes to learn by trial and error only. Therefore I would greatly appreciate, if you could point me to great (language-agnostic) authoritative [books > online courses > tutorials] to learn about the ideas behind these technologies and their actual implementation in detail (javascript, java or any Functional Programming Language).