Nokia’s Open source releases page just got updated with the source code of Nokia 7 Plus. HMD is using software licensed under the General Public License (GPL) and Lesser General Public License (LGPL) which means it has to release the source code used for devices. HMD can provide just selected parts of source code which developers can use to make custom ROM’s for which you’ll need unlocked bootloader. Unfortunately, this is possible only for Nokia 8 which bootloader is the only one unlocked by now.
If you visit Open source releases page you can download the various codes of Nokia 8, Nokia 7 Plus, Nokia 6.1, Nokia 6, Nokia 5.1, Nokia 3.1, Nokia 3, Nokia 2.1, and Nokia 1.