Aamir Khan was incidentally the first star to get his child via surrogacy with his wife Kiran Rao. Azaad, his son born via surrogacy was the first high profile surrogate Bollywood kid.
Shah Rukh Khan and wife Gauri also use IVF and surrogacy to get their latest born AbRam.
Both these stars had opted for gestational surrogacy. In "gestational surrogacy", genetic mother provides the egg which is fertilised with sperm of genetic father but another woman (surrogate mother) carries the foetus and gives birth to the child.
In the case of Tusshar Kapoor and now Karan Johar, both of them are single so they opted for hired surrogates who provided their own eggs, which was fertilised by artificial insemination and the women carried the foetus giving birth the children.
Interestingly Salman Khan is the only unmarried big star among the lot and he was the first actor in Bollywood to play the role of a man who hires a surrogate mother when his wife could not give birth. The film Chori Chori Chupke Chupke was directed by Abbas and Mustan and also starred Rni Mukerji.