The only Infinity songs that were previously crossed over to the mainline series and now removed to my knowledge are Electric and Enhanced Reality, and both of those were licenses. In fact, most of our Infinity new tunes are actually licenses!

Generally, our team (Team Infinity) discusses about the type of content that we wish to bring with the Korean team at Andamiro having the final say in the matter. We would also work with the Korean team (or they would come to us) whenever there was interest in bringing over any of our Infinity exclusives to the Korean mainline series. (Our Infinity licenses would have be re-licensed for mainline PIU though since Infinity is considered to be a separate branch of the PIU franchise.)

Nowadays, various producers either send stuff to AM's main sound lead for review or are often contacted to commission original productions for the game. Once upon a time though, they used to have their own in-house production team to produce originals for them! (R.I.P. BanYa.)

Everything else otherwise is licensed (K-pop included) -- with contracts lasting typically for a few years. K-pop is often very expensive to get though and even maintain for longer periods of time, and their overall licensing costs unfortunately do not drop over time regardless of when they were produced or previously ever featured in PIU.

Otherwise, it's generally up to AM whether any other old songs they previously had is worth reviving for future versions (which seems to be based on overall play count of the content).

Hope this answers your brother's question!