Seeking Lore-How to become a Dragoon

I haven't done many side quests in Ishgard yet so I'm not sure if there is any more information on this than I haven't bothered looking for, but I was wondering if there was any lore behind how to become a part of the Order of Dragoons for Ishgard?

Do I have to be Ishgardian?
Is there some separate trial from the Azure Dragoon trials that I had to do during the Dragoon quest line?
I noticed that they are addressed as "ser" much like a knight would be addressed. Would I have to go through some rank system or apprenticeship in order to become a Dragon Knight?

I did a little research on what an actual dragoon was/is, and as far as I could tell they were specialized knights. So I wonder if maybe the Dragoons in FFXIV work off a similar system of becoming a Dragon Knight.

Any thoughts or help on the subject would be greatly appreciated. If you know of any quests that might have some information please direct me towards them and I will have a look myself. c:

In a couple of cases, the game actually uses the full title Knight Dragoon.

The standard practice, as far as I can tell, is that lower houses rise and fall, form and dissolve, but all lower houses swear fealty to one of the four permanent High Houses, who are officially traced back to Thordan's knights (in truth, all Ishgardians are of those lines if you go back far enough, but this is hidden from most).

From this nobility structure arise knights like you would expect in any feudalism-like system. The dragoons are the elite - the best of the best - who display a mastery of the lance and the acrobatics required to wield it effectively against the dragons. If you're good enough, and swear the oath to protect Ishgard from the Horde until your last, you become Knight Dragoon and a set of Drachen armor and a Gae Bolg are forged for you. From there you're just part of that unit to be used as is best, akin to the Temple Knights under Aymeric, but under the current Azure Dragoon.

You can imagine how it might not be impossible to be an outsider who comes to Ishgard, becomes a knight (see: Lucia), rises to Knight Dragoon, and takes the oath... but it would be a pretty rare occurrence. At the very least, you'd be from Ishgard by then semantically, and certainly of Ishgard from then until you died. That's the oath, after all. Aside from that you just have to be a knight, and the making of a knight is often circumstantial - knighthood comes from all sorts of things, historically - your family's power, great skill, great service, great potential - from the Pillars to the Brume to outside the gates, everyone probably has the potential, rare as some circumstances might be.

The exception to this, of course, is the second Azure Dragoon, you, because Hydaelyn is hax.

Your training is off-the-books. The Azure Dragoon of Ishgard steals the eye and makes off with it, and his guilt-ridden former mentor (the previous Azure Dragoon) hires you to track him down and help clean the whole thing up, if that's even possible. In the midst of this mission, the Eye rouses and bestows its power upon you (the first time it's ever chosen more than one person at a time, as far as any can tell), so the previous Azure Dragoon gives you his soul crystal and sets you on the path because the Eye willed it and he thinks you might be able to help right some wrongs.

In accordance with tradition, you become a great Dragoon, but not a Knight Dragoon; Gerolt even has to reverse-engineer your Gae Bolg. Luckily, Estinien was trying to do the right thing by taking the eye, as Nidhogg has awoken and he's trying to distract him, and after a brief moment of failing to control the Eye (which you apparently never bring up again, because you're classy like that), he vanishes between 2.00 and 3.00 and it turns out he's fine and still leading the Dragoons.

DRG is one of the last jobs I level, so I know little of the post-50 story, but I hear that you actually come back into contact with someone that helped in your early training, the Azure Dragoon's second in command, who now wants tutelage from you (the free, adventurer dragoon) to be a better Knight Dragoon.

As far as I know Knight Dragoons have no connection to the 4 high houses nor the holy see, they are sworn to defend Ishgard from the Horde and that's it, they can't be bought/sworn into a house they directly ignore petty squabbles between the houses unless it threatens Ishgard as a whole.

Plus all of what Master Anonymoose said.

Thanks you guys that actually helped me out a lot. I know a bit about knight hood and I thought if might be similar to that but I wasn't sure if Knight Dragoon was it's own separate knighting.
Thank you so much I'll be keeping all of this into consideration.