Read technology's founding documents
Often times the fastest way to understand a technology at its core is to read the first thing ever produced about it by its creator. Sadly, these papers/demos aren't often suggested as a good introduction to the technology. The modern docs may be up to date, but often are super bloated with all the features that have crept in, or worse, are trying to sell you on a ton of features that aren't unique to it at all.
Have you ever visited a website and it promises to be the "data cloud" or the "deployment operating system"...yes but what exactly is your unique tech. Frequently, the original announcement/paper is easy to read and cuts to the core of the concept.
Take for example the Web. The modern Web has a million topics to learn like HTML, Javascript, servers, databases, etc. But, what is the web at its core? Tim Berners-Lee first presented his idea in this now famous proposal: https://www.w3.org/History/1989/proposal.html.
It even has cool pictures of computers with lightning bolt arrows connecting them (A lot of these founding documents have character and flare to them since they came out raw from the creator).
The proposal discusses nuanced decisions about the web's architecture that we often take for granted, like how hyperlinks should work and why tagging isn't a good way to organize data.
Who would have thought you could learn about what the web is without a single mention of Javascript?
More technologies
Node.js
Ryan Dahl, the creator of Node, gave this presentation back in 2009: https://www.youtube.com/watch?v=ztspvPYybIY. He lays out the new async properties of the runtime as compared to the common blocking runtimes of most server environments. It's not just about JS on the server!
Immediate Mode GUIs
If you've only ever written UI in React, you've been spoiled by having a programming model that lets you simply declare your UI on each render, and not have to worry about updating the underlying DOM. This is effectively an immediate mode paradigm. The alternative, and historically more common one, is "retained mode" where the programmer is responsible for all the stateful and detailed work of managing all the UI objects (adding, removing, etc) on every state update.
Casey Muratori has this fantastic article and a presentation where he (as far as I know) introduces the concept of immediate mode programming as applied to GUIs.
Ruby on Rails
Rails was launched in an era when there weren't any batteries included web frameworks. That was its key contribution, summed up in the original How to Build a Blog in 15 Minutes with Rails live demo. Today, this demo doesn't look like much because almost every language has its own framework inspired by Rails. But before Rails, any one of these features could hundreds of lines of code, and dozens of decisions about code structure, file organization, etc.
The Relational Data Model
In 1969, Edgar F. Codd introduced A Relational Model of Data for Large Shared Data Banks. It describes what we know of today as relational databases. The first few pages are definitely worth a read (after that it gets pretty academic and mathy). It compares the relational model to the other approaches of the time, such as hierarchical and network based models.
React.js
The creators of React at Facebook gave a talk at JSConf in 2013 that spelled out its core ideas. I like how nervous Jordan Walke seems even though he's in the midst of revolutionizing web development.
Clearly these are skewed towards web development, but I think it applies pretty generally. Some other examples you could lookup yourself are: The Unix Papers, Turing (annotated), and Fielding's dissertation on REST. Of course, you'll need the modern docs to get your job done, but these original docs will help you get the overall picture as to why you should use the tech in the first place.