How would i make a union using a script?

Unions and negations can be made through a plugin by using plugin:Union(table of BaseParts) and plugin:Negate(table of BaseParts). UnionOperations can be separated using plugin:Separate(table of PartOperations)

These cannot be used outside of a plugin or outside of studio. Unions cannot be created ingame. If you simply want to join things that are unanchored, you can weld them together:

function weld(a,b)
	local w=Instance.new("Weld",a)
	w.Part0,w.Part1=a,b
	w.C0,w.C1=a.CFrame:inverse(),b.CFrame:inverse()
	return w
end

and they will be stuck together unless something like an explosion breaks them apart.