Some companies, like Motorola and Google, are quick to release kernel source code for their new phones. Nokia is a mixed bag in this regard, especially since it just now released code for the Nokia 2… which came out nearly two years ago.
If you’re not familiar with the process, the Linux kernel used in Android is licensed under GPL, which requires companies to release their modifications. The kernel source package can be used to more easily create custom ROMs and recoveries for the phone, but since the Nokia 2 can’t be (officially) bootloader-unlocked, the source code is more or less just to fulfill GPL obligations.
Nokia publishes kernel source code for the two-year-old Nokia 2 was written by the awesome team at Android Police.