See: Why is there different policies from the USA for Turkey vs Greece/ India?
My above question was closed. So, I am reformulating the question and posting it again.
According to the above link, the Russian S-300 missile defense system is being used by three NATO countries: Greece, Bulgaria, and Slovakia.
According to the above link, the USA is welcoming Greece's decision to purchase F-35.
According to the above link, the USA is worried that Turkey using S-400 and F-35 at the same time would give Russia the opportunity to collect data on F-35. However, we all know that Turkey is already out of the F-35 project.
According to the above link, the USA is soft in India for buying S-400.
In light of the above links and information, What is the basic difference between Turkey using S-400 vs India purchasing S-400 or Greece/Bulgaria/Slovakia using S-300?