How do I make my building system build to the side of an part?

My question is posted here: https://devforum.roblox.com/t/how-do-i-make-my-building-system-that-can-build-to-the-side-of-an-part/1468109

Please Help me so please don't just give me a script without telling me how it works. (Scripting helpers rules)

Already solved on the devforum!