Question

what will hapen if you add buttermilk to chilled milk

Answer 1

If we add buttermilk to chilled milk then the result will be very sour in taste

Answer 2

The chilled milk stays cold for a long time.  It retains the taste of milk.  
The milk takes a very long duration to be fermented and to become curd/buttermilk.   Perhaps it may not become buttermilk due to low temperature.  As fermentation happens better in slightly warm temperatures.