Unity game development questions?

Hello l'm new to Unity and l'm also an experienced programmer recently l dicited to go start learning the ways how create a game with Unity. From start evrything l already know the basic principles of Unity and also know how to setup the player movment also how add camera controls and ather basic things and now dicited to start developing a simple fun game. It's a sandbox type like game and fun building type of game like in Minecraft and l already started developing and l already made the option for how to drag and put cubes in required places but l encoutered a problem well when l in the game when l tested and l found out that when l stack those cubes on top of each ather they don't seem hold still and l wont that players could build their creations like in Minecraft and l want those cubes to hold still and can someone also privide me if some information how you can create an inventory of items in the game?