Where do prefaces come from?

The preface is the prayer spoken or sung by the priest just before, and leading into, the Sanctus.  The number of prefaces in the Roman Missal was significantly expanded in the reforms of the Second Vatican Council.  Where there was formerly just one preface of Christmas (now called "Preface I of the Nativity of the Lord"), there are now three.  Preface II of the Nativity of the Lord was newly composed following the Council.

It is truly right and just, our duty and our salvation,
always and everywhere to give you thanks,
Lord, holy Father, almighty and eternal God,
through Christ our Lord.

For on the feast of this awe-filled mystery,
though invisible in his own divine nature,
he has appeared visibly in ours;
and begotten before all ages,
he has begun to exist in time;
so that, raising up in himself all that was cast down,
he might restore unity to all creation
and call straying humanity back to the heavenly Kingdom.

The preface sounds the principal themes of the Christmas season:  Jesus "is the image of the unseen God" (Colossians 1:15) - God made visible.  His Incarnation is our feast, too, for Jesus becomes human so that we might become like God.

But this "new" preface is based on ancient sources. In an ancient Christmas homily by Pope Leo the Great (c. 400-461), the saint meditates on Christ's Incarnation:

...being invisible in His own nature He became visible in ours, and He whom nothing could contain, was content to be contained: abiding before all time He began to be in time: the Lord of all things, He obscured His immeasurable majesty and took on Him the form of a servant.... (Sermon 22)

The text also borrows from other sources, ranging from an 8th-century preface to the psalms (see Paul Turner's Pastoral Companion to the Roman Missal from World Library Publications for more background on this and many other Missal texts).