It's the same as this question, but the Readied Attack Action is used in your own turn, so all the answers for this other question, as well as its dupes, aren't valid any more, as they are based on the fact that the Extra Attack only works in your turn. This comes from this answer, where this Readied Attack Action is used to hit an enemy while you fall, which arguably wouldn't be possible using the Attack Action. This means you are attacking during your turn using the readied attack.
The Ready Action states
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the cultist steps on the trapdoor, I'll pull the lever that opens it,” and “If the goblin steps next to me, I move away.”
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
The Extra Attack feature states
Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack Action on your turn.
So, my doubt is: being a reaction, does it still count as an Attack Action? The reason I'm confused is that a reaction is not exactly an action, for example, reaction spells can't be used as actions or vice-versa.
Clarifying: I know you can use your readied action in your own turn, which is what is covered here if I didn't miss any thing. The question is about the nature of the attack you are using in your turn. If you ready an attack and use it in your own turn, does it count as an Attack Action or just an attack? I.e., does it trigger Extra Attack (and any other features that trigger when you "take the Attack Action") or no?
Extra argument for the confusion: readying a spell is clearly different from the Cast a Spell action, as it takes concentration and spends the spell slot even if you don't use the spell at all.