bitcoin core development – would you please tell me where is the cpp file in which immature_balance function concerned?

I am just wanting to clear up a few details of the bitcoin immature_balance.Such as how to create the immature_balance? How to increase or reduce the immature_balance? And where is the cpp file which is concerned?
I can’t found the cpp file which is concerned. So would you please tell me where is the cpp file in which immature_balance function concerned? Thank you very much.
By the way , my bitcoin version is bitcoin-0.20 and it runs in segtest mode.