Unity uses a game object based UI system, let's cover how it's done in 3 steps - creating a Canvas, positioning elements and defining user interactions.
Let's create a quit game button. you can use the same step by step approach for any game, it doesn't have to be this specific project I have here.
GitHub repo (full source code): https://github.com/codingLaboratory/U...
Playable demo (Unity WebGL build): https://play.unity.com/en/games/9227b...
Have fun learning game development!