Like when CGP Grey makes a video about the European Union, that video explains everything with details yet clearly and concisely. I thought redux would have been a great word, but it only means "revived"
the essence of a thing in its purest and most concentrated form
you can say:
compendious (adj) compendium (n)
containing the substance of a subject, often an exclusive subject, in a brief form; concise: a compendious history of the world.
marked by brevity and clarity; concise
precis (n, v)
a summary of the essentials of a text; abstract
to shorten by omissions while retaining the basic contents: to abridge a reference book.
v.tr. digest, sum up, put in a nutshell, abridge
n. digest, abridgement, abridged version
adj. digested, summed-up, in-a-nutshell, abridged
Do look into the details, because these are not "perfect" synonyms.
I would call that video "The EU in brief".
In literary forms, a Treatise can often be this kind of work.
I've happened across "cheat sheet" since I asked the question.
One possibility is epitome, in the sense of this definition in The Oxford Universal Dictionary (1955):
A condensed record or representation in miniature.
User49727 suggested this term earlier, but didn't supply the particular meaning relevant to the original question.