How does findFirstChild() know which child is first?

It knows which child you want, because

findFirstChild('ItemHere')

You literally tell it the exact name of the item you want, I suppose if there are multiples of the same item with the same name, then it wouldn't know, so be specific when naming parts, and groups.

findFirstChild is equivalent to the following code:

function obj:findFirstChild(name)
    for _,v in pairs(obj:GetChildren()) do
        if v.Name == name then
            return v
        end
    end
    return false
end

Basically, it looks through the object's children then returns the first one it finds with the matching name. There is no specific one that it will return, unless there is only ONE object named as such.