The Machinas are a race of mechanical humanoid in Xenoblade Chronicles. They are the only known sentient race to have been born from the Mechonis and used to live all over the Titan before they were forced to take refuge on the Fallen Arm after the battle between the Bionis and the Mechonis. They are also the creators of the original Mechons.
The Machinas are a sentient race of mechanical humanoids born from the Mechonis in the same way the Homs, Nopons, High Entias and Giants were born from the Bionis. As a result of their mechanical bodies, they can live thousands of years and are one of the longest-living species in the whole series. They do not need to eat and only require a little bit of water and some Ether from time to time to survive.
Machinas can also attach new mechanical parts to their bodies to augment their strenght or other attributes as seen in some sidequests. Their growth cycle is also pretty particular since Machina children looks nothing like their parents and are basically only a human head in a floating mechanical vessel. It is assumed that they obtain the rest of their bodies by progressively attaching new parts to themselves until they are a fonctioning adult. This process may be dependant on some unknown factors however, as one Machina is known to have growth problems and has been stuck in a child body for way longer than an usual Machina is.
According to them, their body is way more complex than organic ones and Machina get way more often "sick" than Homs for example.
The Machina used to live across all of the Mechonis but they flew when Zanza attacked their capital, Agniratha. Since then they have been living on the Fallen Arm away from the eyes of all life on Bionis. After Shulk recreate the universe, they live alongside all of the other sentient races in the new Colony 9.
Long before the events of the games, the Machina were created by Lady Meyneth in her likeness and quickly became the most advanced civilization in their world due to the sheer complexity of something as vital as their body. They based their religion on their creator, Meyneth, who was in friendly terms with them and met with the less advanced people of Bionis.
The War Between the Titans
The two Titans lived in harmony until Zanza grew fearful of the Machinas, seeing them as a potential threat to his existence, he possessed a Giant named Arglas and launched a surprise attack on them by using the Bionis to wield the Monado and directly attack Agniratha while swarms of Telethias were hunting down the fleeing survivors.
Meyneth retaliated to protect the Machinas and fought against the Bionis using the Mechonis. The two Titans were deeply wounded, Meyneth was forced into a deep slumber and Zanza, still in Arglas' body, was locked up in Prison Island by the inhabitants of the Bionis in hope that a similar tragedy would never occur again.
The Attack of the Mechons
A young Machina named Egil swore to take revenge on Zanza and took over the ruined capital with his armies of Mechons now repurposed for war. Egil planed to use the Mechons to kill as much organic life as he could since he knew it was the only thing keeping Zanza alive. The rest of the Machinas learned the horror of his plan and tried to stop him but were defeated and forced to exile on the Fallen Arm where they hid their existence from the people of Bionis in fear of a retaliation for Egil's actions. In the following years, the Mechon constantly attacked the Homs settlements who were the easier target for Egil's armies compared to the advanced High Entias and the hard to reach Nopons. This lead to the discovery of the Monado who was hidden by the ancestors of the High Entias in the Valak Mountain to restrain Zanza's soul, which was contained in it. Eventually Shulk swore revenge against the Mechons for attacking his land and seemingly killing Fiora, which led him to the Fallen Arm where he learn of the existence of the Machina.
Meeting with the Machinas
The party meets with the Machina after accidently falling from Galahad Fortress as a result of the battle between Egil in his Faced Mechon, Yaldabaoth, and Fiora. There they are scattered around the Fallen Arm and eventually regroups in the Hidden Machina Village where they are introduced to their leader Miqol while Fiora is given proper medical treatment by Linada, their doctor.
Miqol reveal to them that he is Egil's father and that he wants the party to kill his son to end the suffering of the world. Miqol also tell them to climb up the Mechonis to find Vanea, Egil's sister, who would be willing to help them stop him.
The Second War between the Titans
After meeting up with Vanea in Mechonis Field, the party manage to reach Egil and fight him, they are however defeated by Yaldabaoth and after being saved by Gadolt and Meyneth, they withness Egil reawakening the Mechonis and attacking various parts of the Bionis. With the help of Miqol and the Machinas, they manage to reach the Mechonis Core where they defeat Egil only to withness the return of Zanza and the reawakening of the Bionis. The two Titans fights but Egil is quickly overpowered by the God and so is Meyneth. The party is forced to flee while a dying Egil maintains the control of the Mechonis until its complete destruction to protect the party. The rest of the Machinas are largely unaffected by the destruction of the Mechonis as they were already living on the Fallen Arm.
In the ending, the Machina can be seen living in peace in Colony 9. Dunban remarks on the advanced technology the Machina are sharing with the colony.