The Tibetan empire begins as a confederation of clans in the Yarlung valley. Songtsen Gampo came to power after his father was poisoned, put down the revolts that followed, and by the 630s was fielding armies far enough afield that the Tang court had to treat him as an equal. He moved the seat of power to Lhasa, imposed a system of administrative districts and military units on territories that had been clan holdings, and set up the taxation and record keeping that made the later expansion possible.
The best evidence for all of this is Tibetan and close to contemporary. The Old Tibetan Annals, recovered from the sealed library cave at Dunhuang, are year-by-year court records, and the Old Tibetan Chronicle preserves narrative and song from the same world. Beside them stand the Chinese dynastic histories, written by a state with every reason to pay attention to what was happening on its western frontier.
His marriages are part of that political record. The Tang princess Wencheng was sent to him in 641 and the alliance is documented on both sides. A Nepalese princess, called Bhrikuti in later Tibetan sources, is not attested outside them and her historicity is questioned. The two brides carry a great deal of weight in later legend, because each is said to have brought a Buddha image with her, and those images are what the Jokhang and Ramoche temples were later built to house.