One of the strongest points of using the Android OS is on the infinite possibilities and customizations that one can achieve with a smartphone. Through Custom ROMs, many enthusiasts and developers can experiment their phones out of the OS-imposed by manufacturers.
Some companies like Motorola and OnePlus always allowed developers to unlock their bootloader, in order to start the development by flashing Custom Recoveries or ROMs. Other companies like Xiaomi have more strict policies, only allowing users to unlock bootloader after a determined period of time. However, some few companies like Nokia never provided a way for a developer to unlock their phone. From today, things will change.