There are several features with phrases similar to "When you take the Attack action" and I am wondering when exactly you have "taken the Attack action", I have found several questions and features which have ended up making this even more confusing than I originally thought.
Extra Attack says:
"You can attack twice, instead of once, whenever you take the Attack action on your turn."
This implies the following is true before getting extra attack: "you can attack once whenever you take the Attack action", which, to me, is equivalent to the following: "After you take the attack action, you can attack once" which means you would have to first take the attack action, and thus you are allowed to attack once.
And then the Fighter's Extra Attack (2) says:
"You can attack three times whenever you take the Attack action on your turn."
I can't think of a way "taking the Attack action" could mean making every available attack while having the extra attack features still work as intended.
The Battle Master Fighter's Commander's Strike Maneuver states:
"When you take the Attack action on your turn, you can forgo one of your attacks..."
If taking the Attack action meant making an entire attack then this maneuver would be useless until you acquired extra attack because you wouldn't have any attacks left that you could forgo.
Then there's the Polearm Master feat which says:
"When you take the Attack action and attack with only a glaive, halberd, quarterstaff, or spear..."
This unfortunately seems to say that "taking the Attack action" means completing all available attacks because only at that time could you know whether all your attacks were made with the required weapons.
And then there's the Way of the Four Element Monk's Fangs of the Fire Snake feature which says:
"When you use the Attack action on your turn... Your reach with your unarmed strikes increases by 10 feet for that action, as well as the rest of the turn..."
I would assume this is meant to benefit the triggering attack as well, in which case you would have to gain this benefit before you've made your first attack, which means "taking the Attack action" would happen before your first attack.
On top of all this, there's the Gloom Stalker Ranger's Dread Ambusher which has the following:
"If you take the Attack action on that turn, you can make one additional weapon attack as part of that action..." Emphasis mine
This answer here says that the additional attack must come after your first attack, due to the part that I have bolded making an exception to the general rule.
Lastly there is the Monk's Flurry of Blows feature which states:
"Immediately after you take the Attack action on your turn..."
This answer here states that you can use FoB anytime after the first attack of your Attack action. But I am not exactly sure how this works, especially with the "immediately after" bit.
And amongst all this, though his tweets are no longer official, Crawford has said the following:
"No general rule allows you to insert a bonus action between attacks in a single action. You can interrupt a multiple-attack action with a bonus action/reaction only if the trigger of the bonus action/reaction is an attack, rather than the action."
Which implies that "taking the Attack action" would actually mean making every single available attack.
All of these different features, at least to me, don't seem to have any coherent/consistent meaning of the phrase and I'm wondering if there is one supported in these books.
When have you actually taken/used the Attack action?
This matters because at what point you are considered to have taken this action determines when you can use any of the above features.