Author's Note: Okay meant to post this earlier, also meant to write more of these. Again been busy with the book. But now I'm posting this and I'm putting the book on hold as I do some more write ups. So I ask for Patience.
Overview
The Quarian's represent the first of the so called 'citadel races' that we can say that we have had a proper, friendly relationship with, their creations the Geth the second. Unfortunately the first are also probably the weakest of the various races that we could've gotten in contact with, the Geth more promisingly seem to have enough military power that the council hasn't attempted to wipe them off the face of that galactic map as of yet. See our write up for more details.
Biology
Quarian's are what's referred to as 'Dextro Amino Protein' based species or DAP as opposed to most of the rest of the galaxy who are DNA based. According to the Lich's what this means in layman's terms is that the Quarian's are bio-chemically incompatible with the rest of the Galaxy. According to the same scientists this is actually far more expected than everyone just having DNA, and also implies that the rest of us share a 'common ancestor' if you can call primordial soup of bacteria a 'common ancestor' that is. Moving on. Quarian's are classified as a humanoid race with eyes able to see into the ultra-violet spectrum with three fingers including a thumb. The most notable aspect of Quarian biology is their extremely weak immune system, apparently stemming from a combination of naturally evolving alongside a type of symbiotic Bacteria along with spending the past three hundred years inside of a fleet of migrating ships only ever stopping planet-side for their infamous 'pilgrimages' more on that below. The presence of this unnaturally weak immune system necessitates the use of the omnipresent suits that they wear everywhere which is designed to isolate them from the rest of the environment and all the unpleasantness it has to offer. Aside from this weakened immune system the Quarian physiology is surprisingly robust with Bio-medical scans indicating that a 'healthy' Quarian can survive across a variety of different environments and even on certain high gravity worlds.
The Geth are a race of networked VI programs inhabiting Synthetic bodies originally created as servants of the Quarian's. They mostly have the same body types with exception to when they decide to assume more 'purpose' built platforms. One odd quirk of the Geth is that their networked VI programs require that multiple programs be loaded into a single platform, most platforms have a hundred or so programs loaded. However in order to achieve enough intelligence to have a conversation 1183 programs have been loaded into a single platform dubbed Envoy. If a physical body gets destroyed they tend to upload programs into the nearest platforms like towns accepting refugees after a natural disaster. An odd thing to note is that these 'town bodies' are something of a democracy. In order to take a course of action the programs inside must first reach 'consensus' that is to say the majority of the programs involved in said body must first agree that a given course of action is ultimately for their best interest. That said, a individual program of the platform shouldn't be treated as being intelligent enough to warrant being treated as a individual, it's only when they pile together into one platform in the triple digits that the Networked Vi's gain sentience.
History
Quarian's emerged from their home world of Rannoch at roughly the same time as several other races throughout the Galaxy at which point they made the journey through the Mass Relay's to the council. Little over three hundred years ago the Quarian's attempted various experiments with Networking VI programs in order to try and bypass strict council law's forbidding the use of the more intelligent AI programs, as to why in the Quarian language Geth literally means 'Servant.' It should be noted that the Geth weren't created out of a sense of laziness, unlike some slugs that I can think of, rather they were simply created to do the jobs that were too dangerous for a organic to perform. The Council did not appreciate the spirit of the project and so ordered that the Geth be shut down immediately. We're given to understand that a short civil war broke out between two factions of Quarian's one trying to follow the Council's orders to shut the Geth down and the other trying to save them. The Geth themselves, unable to process the violence and in a state between mortal terror and Panic did the only thing they could think of that would work and opted to chase their creators away and in doing so united them against a common enemy that being the Geth themselves.
The Quarian's have lived in exile aboard their live ships ever since. The Council viewed the Geth as a massive mess nuisance that they were now stuck dealing with and so decided to punish the Quarian's by revoking their embassy aboard the Citadel and forbidding them from colonising a new world. The Geth on the other hand were left on the back foot, they were now servants without masters and so for a significant portion of the next few centuries were left twiddling metal thumbs unable to think of what they should be doing. Of course the Council, and the Quarian's, kept them busy, making repeated attempts to annihilate them and as a consequence the Geth would focus their efforts on making surprisingly advance defensive and offensive technology in order to fend off hostile forces, to this day the council maintains a massive bounty for any Geth technology that can be brought to them intact, to this day no one has been able to claim this bounty.
More recently at the recommendation of Cherrel Kahn a Tebrid by the handle of 'KrakkenRipper337' and his Custodian negotiated if not reconciliation between Geth and Quarian then at least a ceasefire. The Geth have completed a hand over of the Quarian home-world in exchange for help with their own project in which they are planning on building a Dyson Sphere as part of a attempt by them to achieve their full potential while the Quarian's are currently helping Union scientists to develop a serum to fix their failing immune systems. A second joint Quarian/Turian colony is currently being prepared in the Eden system of space under the watch of the Custodians, given both sides completely justified and culturally indentured anti-synth culture this is completely understandable. Currently the Custodian's are finishing work on the Dextro-Amino based bio-domes one the moon of Eden Prime known as Eden B and once confident that the two factions are capable of maintaining the facilities on their own turn them over to their new owners.
Military
Quarian's don't have dedicated military forces, they mostly have Quarians who went to Turian worlds for their pilgrimages and thus learned how to shoot and fight.
The Geth on the other hand maintain a variety of forces.
The most infamous and unique example is the 'Leapers' who tend to jump from place to place, sticking to the side of pre fabricated buildings.
Another notable example is the Geth Armature a massive quadrupedal unit armed with large Plasma casters to fire at potential targets.
Aside from the Above two examples the bulk of the Ground forces consist of the boring but Practical trooper and the slightly larger Geth Prime's who act as centralised command units.