The official Open Source Code page for Nokia smartphones got updated once again. Now the source code is available for HMD’s latest phone, Nokia 8.1. If you are interested in peeking in this code, you can get it by going to Open Source Code page and download it there.
If you are in the whole custom ROM building thing, this won’t be helpful since the bootloader is still locked for almost all Android running Nokia phones. Until then, you can pile up source codes of many Nokia phones.