Or worded differently: once i establish longer-lasting facts through narrative or 'create an advantage', are they aspects beyond the scene? If PC finds an item, for example, or some character aspect should change in the light of the narrative because it is no longer true. If something about the character is established in the narrative, should it be part of that characters aspects?
[RPG] If aspects are facts, are all facts aspects
Aspects determine what is important in the scene. Those scene aspects defined when the scene is framed are the ones that the GM deemed to be important in the scene; the ones that add flavor to the narrative.
However, narrative games like fate share the narrative direction with the players. In this particular case, though the GM didn't deem the fact that the marketplace was crowded one of the things that would add flavor to the characters' interaction with the scene, one of the players did. And because of that, and because he felt strongly enough about it, he took an action or spent part of his narrative capital (Fate points) to bring it into play.
As a GM, never feel that you have to define everything- only those things that are important should be defined before hand. And as a player, never think that you are confined by those things the GM has declared- Fate points and your skills give you the tools that you need to help to expand and drive the narrative.
To directly answer your final question, i.e. "I understand that Create an Advantage is about not only creating aspects, but also getting a free use. Does one have to Create an Advantage to take advantage of aspects that seem implicit, but were not yet declared?"
Creating an aspect is one way of getting the use of an implicit aspect. In this manner, you get a free invocation. However, if you're not looking for a free invocation, i.e. you just think that it's important to note that the marketplace is crowded, you can suggest it freely, and if everyone agrees, the Crowded aspect is added. This is stated in the Fate Core Rulebook on p. 78. It can also be found in the Fate Core SRD- Creating and Discovering New Aspects in Play.
Then later, if you, or someone else, wants to use it, a Fate point can be spent as normal to invoke it.
You're gonna have to go to the Fate Core book for the full explanation. Although they don't mention it for this particular case, FAE makes a habit of expecting players to refer to Core for more details on a host of subjects. This is because Fate Accelerated, in order to stay svelte, doesn't talk much about corner cases. Since multiple free invokes on one aspect usually come from "with style" outcomes on Create Advantage, that's where they put this detail. They may feel it's implied on page 28, but it could definitely use more explication.
So for more comprehensive coverage we look at page 70 of the Fate Core book, Free Invocations:
Free invocations work like normal ones except in two ways: no fate points are exchanged, and you can stack them with a normal invocation for a better bonus. [...] You can also stack multiple free invocations together.
It's a shame they left these lines out of Fate Accelerated, but comparing the two books' phrasing we can easily infer that they had this rule in mind. They just pared it down a little too aggressively.
So the answer is: Free invokes, as part of their inherent "freebie" nature, ignore the "one invoke per aspect per roll" rule, stacking with each other and with normal invokes. Put another way, you can invoke an aspect as many times as you like for the same roll provided a maximum of one of those invokes is made using an actual factual Fate point.
This means you can stack one normal invoke (powered by a Fate point) with as many free invokes as you have available on the same aspect and get the full effect of all of them.
The short answer is yes. It is because of the Bronze Rule i.e. the Fate Fractal on Page 270 of the Fate Core rulebook.
The trick is to describe the elements you are dealing with in natural terms first. Look for logical subgroups or subelements. For example Super Porsche, a car with super tech, can be described as a character in its own right. In addition the Super Porsche's amazing engine can also be its own character.
Now you ask about aspects specifically. I like to think of aspects as packages. A package in a generic system (Fate, GURPS, Hero, Savage World) a bundle of traits (attributes, advantages, flaws, gifts, skills, etc) that describe an element of the setting. A campaign revolving around Navy Seals would have different packages for the different MOSs.
Now aspects in fate are natural language descriptions of some element in the setting. A Navy SEAL demolition specialist would have a demolition specialist aspect with benefits and limitations. A Super Porsche Amazing Engine would have an aspect of requiring Unobtainum as a fuel source with own attendant set of complications. The fact that a characters is from New York's Hell Kitchen can be an aspect with benefits and complications.
Remember per the Fate Core rules on page 56 an aspect is
Unique or noteworthy is why you don't need to list every attribute or bit of information about a thing or character as an aspect. Only the things that are important to the campaign should be made aspects.