Alter Egos are High Servants created as complexes of Heroic Spirits.
BB accessed the Moon Cell's Servant Archives, chose goddesses compatible with the Ego from them, and reproduced the data. Alter Egos came into existence as complexes of goddesses.
Meltlilith is made of three goddesses.
The first is the pure virgin goddess Artemis of Greek mythology. She would unfailing shoot through, simultaneously granting disease and death, those who sought to attain the magical power which pulls the tides and causes them to rise, in other words, their link to the moon's movement.
The second is the cruciform snake staff which originates from Leviathan, who appears in the Old Testament or maybe Lotan from Ugaritic myths. It is something with the same origin as the staff that Moses used to split the Red Sea, while also acting as a symbol of the sea that that staff split. Its owner will be granted a transformative property like that of water, making it feasible for them to adapt to any situation. Furthermore, their form will be able to change phantasmagorically, and it would even be possible to take the shape of the target of one's envy.
The third is the river goddess Saraswati. She holds the ability to control "things that flow," such as natural phenomena, among which is water and wind, music and words, speech, poetry and prose. The timbre that flows from her can excite any emotion in the heart of those who hear it and, moreover, can interfere with the flow of electricity in a computer network.

Fate/EXTRA CCC Meltlilith (Alter Ego) Matrix