According to Encyclopædia Britannica:
The Azerbaijanis (peoples of the Republic of Azerbaijan) are of mixed ethnic origin, the oldest element deriving from the indigenous population of eastern Transcaucasia and possibly from the Medians of northern Persia.

There is evidence that, despite repeated invasions and migrations, aboriginal Caucasians may have been culturally assimilated, first by Ancient Iranian peoples and later by the Oghuz.
Considerable information has been learned about the Caucasian Albanians including their language, history, early conversion to Christianity.
The Udi language, still spoken in Azerbaijan, may be a remnant of the Albanians' language.
According to Encyclopædia Britannica:
The Azerbaijanis (peoples of the Republic of Azerbaijan) are of mixed ethnic origin, the oldest element deriving from the indigenous population of eastern Transcaucasia and possibly from the Medians of northern Persia.
There is evidence that, despite repeated invasions and migrations, aboriginal Caucasians may have been culturally assimilated, first by Ancient Iranian peoples and later by the Oghuz. Considerable information has been learned about the Caucasian Albanians including their language, history, early conversion to Christianity. The Udi language, still spoken in Azerbaijan, may be a remnant of the Albanians' language.
Azerbaijanis