I think it just depends on the wrestler, the wrestler's connections, where they seen them on the main roster, how seasoned they are, where there is a need on specific shows, etc...
I don't think there is one specific way they do it. Although Dom and Fatu can be examples of nepotism, they both made immediate impacts on the main roster that would have been lost in NXT.
Stephanie Vaquer was asked if she wanted to go the Main Roster and she chose NXT. I don't think that offer was given to Giulia.
I think it all depends on the wrestler. Fatu and Dom being part of wrestling families definitely helps but I'm not going to take anything away from them because they have put in work, especially Dom. Only a few years ago cats were comparing him to Sammy Guevara and saying he was the worst pro wrestler in WWE. He had mega heat that damn near flipped to mega pops last night.