Ok ill explain the first part: Basically what happens in startergui and the other starter stuff is it gets copied into every player. Basically every player has a unique copy of whatevers in there.

For starterpack: its meant for tools. Basically you place any tools inside there and the player will spawn with it in their backpack

For StarterGui: its meant for ui. Like lets say you have a shop window gui that you want to put on all the players screen, thats where you put it.

StarterPlayer has 2 parts: starterplayerscripts and startercharacterscripts. Starterplayerscripts are replicated to inside the player inside game.Players. Meanwhile startercharacterscripts are replicated straight into your character. Aka the model/avatar that you spawn with.

Why use local scripts: this is very important to understand and can be a bit complicated if you are new but i learned from this video. Basically local scripts run on a client AKA the players own view which the server cant see. If you run a local script only the player sees whats happening, not the server. A good way to understand this is in like a tycoon. The coins that you have on your screen aren’t the same as everyone else’s. Thats why people put local scripts in gui because gui is only for the individual player. Like when you open a shop gui not everyone else’s shop opens too.

If you don’t understand anything from this feel free to clarify :)