I run cyanogenmod on my nexus 6p and monthly need to update the vendor, radio and bootloader partitions. Below are useful links and the fastboot commands.
download image from https://developers.google.com/android/images
extract angler-xxxxxx-factory-yyyyyyyy.zip
cd angler-xxxxxx
extract image-angler-xxxxxx.zip
fastboot flash vendor vendor.img fastboot flash radio radio-angler-angler-02.50.img fastboot flash bootloader bootloader-angler-angler-02.45.img
to upgrade twrp download from
https://dl.twrp.me/angler/
fastboot flash recovery twrp.img