So for example, I have a building system. I don't know how to save these to the cloud. Could someone please explain how to use datastores for this?
Datastores will only allow you to save simple values like numbers, bools, and strings. Since they cannot save complex values like a BasePart, we extract all the properties we need to remember from the part and save them into arrays of stings/numbers. When loading a saved building, we take the array of properties received from the datastore and put them into objects. This process is called "Serialization" and you can learn more about it here:
https://devforum.roblox.com/t/how-to-save-parts-and-the-idea-of-serialization/524311/3