This is for the few Nokia 6.1 Plus users who have unlocked their bootloaders. Maher Abed, Senior Member at XDA has developed a version of TWRP for the Nokia 6.1 Plus. The version of TWRP is compatible with Nokia 6.1 Plus devices running the August security update. The test was conducted on the Indian variant. If it does not work on your variant of Nokia 6.1 Plus then you can get in touch with Maher.
For download links and the steps to flash it, please follow the XDA thread.
Maher nad Dhruv have also made TWRP versions for Nokia 6.1, Nokia 7 Plus and Nokia 8. Be sure to check their work if you happen to have a different device.
As for bootloader unlocking, an unofficial paid method is available which was patched with the August update. If you want to unlock then you can switch the active slot to revert to July and unlock the bootloader. It will not be locked by any further OTA updates. Do note that the slot change method will only work if you have not installed 2 August updates.
Nokia Mobile has been speechless about the official bootloader unlock ability for Nokia phones. There’s a petition going on at Nokia community forums and you should spare a few minutes to vote there.