After completing my b.arch course can i get placement in companies like mahindra, audi .... as an automobile designer ...If yes, how

As per placement is concerned after doing B.Arc you will definitely get placement as per your capabilities  and ability to think in desigining and in any other field required to these companies and they train you but it's not your passion play .  Since your main domain has no role to play in such automotive company .

Either you can do M.DES from iits or nits or other renowned institute after writing CEED EXAM . And enter into their car designing , or industrial design sectors.

As architecture is more required branch in civil sectors.

Google few of reputed indian architects : Hafeez contractor, B.V doshi, HCP ahmedabad, Abhimanyu dalal, Hiren Patel , Zafar chaudhary, Contractor Didi, Late Charles correa.