The initial penetration of Turkic tribes into the Cis-Urals and the Volga region dates back to the 3rd-4th centuries AD and is associated with the era of the Hunnic invasion and other nomadic tribes' incursions into Eastern Europe. Settling in the Cis-Urals and the Volga region, they adopted elements of the culture of the local Finno-Ugric peoples and, in part, mixed with them. In the 5th-7th centuries, a second wave of Turkic tribes' advance into the forest and forest-steppe regions of Western Siberia, the Cis-Urals, and the Volga region is observed, associated with the expansion of the Turkic Khaganate. In the 7th-8th centuries, Bulgar tribes came to the Volga region from the Azov region, conquering the existing Finno-Ugric and Turkic tribes (including, possibly, the ancestors of the Bashkirs) and in the 9th-10th centuries created a state – Volga-Kama Bulgaria. After the defeat of Volga Bulgaria in 1236 and a series of uprisings (the uprising of Bayan and Jiku, the uprising of Bachman), Volga Bulgaria was finally conquered by the Mongols. The Bulgar population was displaced to the north (modern-day Tatarstan), replaced, and partially assimilated.
In the 13th-15th centuries, when most Turkic tribes were part of the Golden Horde, there was some transformation of the language and culture of the Bulgars.
and another website:
http://bulgar-portal.ru/