Why do Players HAVE to collide with npcs?

I have the script that makes players not collide with npcs but I don't know where to put it nor how to make group collisons. Can anyone tell me where to put the script and how to make collison group for npcs?

local PhysicsService = 
game:GetService("PhysicsService")
npcGroup = "npcGroup"
PhysicsService:CreateCollisionGroup(npcGroup)    PhysicsService:CollisionGroupSetCollidable(npcGroup, npcGroup, false) 
local function setCollisionGroupRecursive(object) 
        if object:IsA("BasePart") then 
            PhysicsService:SetPartCollisionGroup(object, npcGroup) 
end
    for _, child in ipairs(object:GetChildren()) do 
        setCollisionGroupRecursive(child)
        end
end
npc = workspace.NPC
setCollisionGroupRecursive(npc)