So I want to make an NPC that only moves when the player (character) isn't looking, like how SCP-173 models on Roblox do this. Do we have to use any services or stuff, and just show me some example code for this or link me to a tutorial as I couldn't find any.
Welcome to Scripting Helpers nashdaar1!
What I would do to answer your questions would be to study 173 assets in the inventory and try to reverse engineer their scripts. While doing this, it might be a good idea to use the ROBLOX API to assist you to recognize what functions do.
I hope this answers your question!