This is the most critical step. Nokia devices are notoriously difficult to unlock compared to Xiaomi or OnePlus. You cannot simply toggle a switch in developer options and run a fastboot command.
TWRP (Team Win Recovery Project) is a custom recovery environment. You cannot install a custom ROM using the stock Nokia recovery. You will need to find the specific TWRP image file for the Nokia 8.1 (codenamed PNX or CTL depending on the variant).
Installing a is the single best way to breathe new life into your aging device. It allows you to upgrade to the latest Android version, remove bloatware, improve battery life, and unlock features that the manufacturer never provided.
This is a high-level guide. Always consult the specific ROM’s XDA thread.