Button to assign gameobject to be placed in world

Hello, I'm still new to C# programming and so I'm kinda lost as to where to start with what I need my GUI to do. I'm not looking for code, I'm just looking for either a tutorial that will guide me so I can learn this stuff OR at least what type of functions I need to be exploring/learning to make this happen. I've been scouring the internet for a couple weeks now and I'm still confused as to what I need to look for.

Here is my issue:

I want to make a list of options on the bottom (assignable based upon the player's inventory) that the play can select with the number keys. The selected gameobject will then be able to be placed in the game world. So, to list what I'm looking for:

1) How do I make a panel/buttons along the bottom that will update based upon what the player drags/drops onto them from their inventory?

2) How to I make it so that the player can select what object they would like to place by using the number keys on the keyboard?

3) How do I make it so that the player will place the selected object in the world where they are pointing at when clicking the mouse?

If it helps, the GUI will be essentially setup like Minecraft in creative mode (not building a Minecraft clone, but the "level editor" I'm building will function essentially the same way).