Honestly, it isn't "newbies" and I am not the biggest fan of you using that term.
There are newer stars who have made waves like Lars Sullivan, and the Velveteen Dream, but none of them is by any means "newbies." Velveteen Dream, aka Patrick Clark, did wrestle elsewhere before being signed after being on "Tough Enough." I mean, there are also former athletes who are newer to wrestling, but even then, they often work the NXT Live Event scene and work in the performance centre to work on their in-ring ability and character for some time before they even appear on NXT TV, to the point where they aren't even "newbies" anymore. They still will likely need to develop more.
Yet in the same breath, there are quite a few wrestlers, a large amount of them, who are well versed in the independent scene and around the world. Johnny Gargano is a former Dragongate Champion, Adam Cole (BAYBAY) is a three-time Ring of Honor Champion, Aleister Black wrestled all over Europe and the US even as "Tommy End" before going to WWE. Then we have EC3, who is a former TNA World Champion after leaving WWE originally, with him finally returning this year. Eric Young, who was in NXT and is debuting on the main roster, is a nearly 20-year veteran of Wrestling and is 38. Bobby Roode, another former NXT Champion, is 41!
NXT is meant to be WWE's "developmental territory" but due to the outstanding quality of the roster and the shows themselves, it is almost like WWE's third brand unofficially as their weekly TV is typically pretty good, and the Takeover events, which are their "PPVs" often overshadow the main roster shows of the same weekend.
If you're referring to AJ Styles, then he skipped NXT and went to the main roster. If you're referring to AJ Lee, the former female superstar, then she debuted on the old NXT where it was treated like a competition show like Tough Enough. That was hot garbage. The NXT the way they have it now is honestly very good.