Applying a chroma key effect based on a black and white video

Hi - hopefully this makes sense. I downloaded an iStock video that the first half is in full color, second half is the same video but colors removed to have a strong black & white contrast so you can key out the ground.

What is it called to apply a chroma key effect to a stock video based on an identical stock black-and-white video? I want to key out "the black" in the high contrast portion of the video and apply it to the color version.

if someone could let me know what that's called I be so grateful. I'll look up tutorials but for the life of me everything I search leaves me empty handed.

this is my video I'm working with, if helpful.

https://www.istockphoto.com/video/christmas-frame-or-new-year-frame-with-fir-or-pine-leaves-and-snowflakes-in-gold-gm1423280271-468309085