Well Iheanacho is a signing, as is Roberts, most of the rest have been mainly recent addittions as the poster you were replying to has suggested. The ones you have missed out are the ones I'm referring to & would rather not single out.
i think it's down to injuries rather than any Pep influence, but in most games I don't think many of the kids on the bench are there to play, under any but catastrophic circumstances good or not.
You have seen Manu Garcia play, for example. Do you really think he was getting on, away at Stoke, in a hurricane ? If the roof fell on the others perhaps.
It's clear Manu is seen as an exceptional talent, and Pellegrini likes having them in the first team squad to get used to the match day experience. I expect if he hadn't been injured he'd have seen Celina's game time in the last few games.
So if he wasn't going to play vs Stoke, then it was to get him used to being in the first team which will benefit him when he actually did play. It's also used as a reward for the youth players, and you can see from their reactions on social media it means a lot to them, a sign they are on track.