In a country like India, where mostly are hindus.
If I go to public places like in trains, etc. should I consider everything najis? What things can be assumed najis or pak ( like floor, wash basins, taps, seat (if gets wet), etc

Not everything in India is Najis. Surface or place is regarded as Najis when it is touched by Hindu with wet hand. One has to be certain that it’s touched and not speculation