Part of what keeps Android so open for development is the General Public License (GPL), a license under which the Linux kernel is under. In short, Android smartphones all run the Linux kernel in some way or another, meaning that they must abide by the GPL, which states that modified source code should be published in readable format once it's compiled and distributed. Some manufacturers don't abide by the GPL, with Xiaomi being notorious for breaking it back in the day. Some manufacturers have tried "creative" ways around it, but they still are breaking the GPL. HMD Global has finally released the kernel source code for the Nokia 7 Plus as well as the newly announced Nokia 8.1.
The kernel source code is a near-essential component in custom ROM and kernel development and is often a precursor before developers can get working on tinkering with their smartphones. There's only one problem when it comes to Nokia smartphones, however, and that is to this day, the only smartphone released by HMD Global that can be unlocked is the Nokia 8. It's great that HMD Global is upholding the GPL, but it doesn't mean a huge amount for custom development.
Only in April of last year did HMD Global start releasing the kernel source code. Back then, the company was widely known for not releasing kernel source code and actively going against the development community. While the company still is doing that, releasing kernel source code is a step in the right direction for sure. HMD Global once said that they would unlock the bootloaders of devices "one model at a time", yet we've only seen one unlocked thus far. It's possible that the company may eventually unlock more devices, but for now, there's not a huge amount that can be done with the kernel source code. If you'd like to check it out anyway, you can check out the links below for the Nokia 7 Plus and the Nokia 8.1.
Nokia 7 Plus kernel source code Nokia 8.1 kernel source code