Let's start simple, after playing The Beginner's Guide you gain more knowledge about the lore, the characters, etc, by playing through the game again, you find deeper meaning and stuff you wouldn't know without already completing the game, like how the game is fictional

Unlike The Stanley Parable, this game lets you turn on sv_cheats, so you can use noclip to find some interesting stuff, I'd recommend exploring locations like the hidden rooms in the puzzle map or out of bounds in the Machine map

This command removes things, you can do stuff like skip triggers (next section of guide), find the "unrefreshed void" (try removing the door at the starting location in machine), get to some areas early without noclip (like the prison cell in the Island map), and do... other stuff

Triggers are invisible shapes that make the things do the thing when the player walks into them, you can make them visible with a console command ("showtriggers_toggle" or something similar), you could try some challenges like avoiding as many notes as possible in the "notes" map and getting through the invisible maze in tower (yes there's actually a maze and you're not surrounded by death barriers), you might also find something hidden, or you might not

Similar to The Stanley Parable, your playermodel is a gliched version of Chell (the Portal 1 version), try playing the entire game in thirdperson, it can be challenging at times since most of your PM is located to the left of the camera, making it hard to know where you actually are

And to end this guide, we have the "map" command, this lets you go to any map by adding the name of the map after the word "map", you can combine this with noclip to do stuff like explore the maze at the end of the game and quickly go to previous maps without going to the main menu