@SpiffyLet me add my bit to it as well, based on what has been said IC up to now.
First, Hohenheim, the Alchemist of Genesis, claimed that she was the creator of Nexus' humanity (presumably based on humanity from outside of it... probably?) but, for some reason went on a self-imposed exile, leaving them to fend for themselves. While people should take Hohenheim's words with a huge grain of salt, it's also very obvious that she has the power to back it up. Her reasons, and the truth about what happened are yet to be seen.
After mankind was left on their won, they found that fighting the Angels and Demons, not to say even surviving on the very aggressive land of the Nexus was a really harsh task. They were the underdogs being kicked by everyone for ages, even with their rising technology, it was harsh to get an edge over the other factions.
It was then that Eos came in, like a literal Messiah and gave them the power and technology that they enjoy today. With this, Mankind (or better, their Machines) are finally able to fight off the other faction at an equal footing. Eos' origins and history are a mystery to all but herself and a very select few, the only thing that's know was that she was found/met for the first time in some ancient ruins.
Eos' technology and power is in a totally different level than that of the other Machina. Even eons after she has been leading them, no scientist was ever able to crack down how or why Eos can even operate. On a hand she looks like she was made using super outdated tech (as Hohenheim said once IC, Eos was made in a world where the most advanced tech available were smartphones), on the other she is so complex that no one can be even fathom what her creator was thinking.
The part about she having a Sun Goddess', more specifically Amaterasu, heart as her power core (the reason why it's called Amaterasu Drive) is known by some members of the Machina and seems to be the key point of why she's so different from everyone else and why Eos is extremely deadly to Demons, in fact, some theorize that she was built as a Demon Hunter android. She doesn't make secret of this part of her, but also doesn't go out of her way to tell it to anyone.
Oh, and for some reason, Eos is also a literal idol among her people. Her performances don't seems to come from being made a music machine, it's more like she actually has a soul behind her singing and eventual guitar performances.
Anyway, this is pretty much all the information regarding Eos and the Machina that have been said IC up to now. You are welcome to look into more of it in case you still want more answers.