All the details of the deal between YRF and Star have been discussed for months, since October 2019. In short, YRF was unhappy that its exclusive satellite partner was neglecting its movies, so it walked out of the decades-long relationship and struck a deal with Star. New films from War onwards will be with Star, but existing films until Thugs of Hindostan will continue to be with Sony.
After SPN lost YRF it (specifically Sony Max 2 and Sony Wah; Sony Max, not so much) has been laying more emphasis on Hindi movies, but SPN may lose other studios too and it is quickly running out of time, as
@dino29 and
@ak7777 bros can explain better than me.