A shooter guy playing Overture

[Googolplex]

It adds a lot.

Oh, I'm really surprised! Can you say me, what will be added?

Aren't all the demos 1 (or 2) full levels of the complete game?

There is only one demo called "Tech-Demo", which is full playable and shows some technics of the following official game.
The official full game was released a year later and has nothing common with this demo.

Then there are "normal" demos of each game episode (Overture, Black Plague and Requiem). But they don't have the full levels, they are very short.

Play the full game, only this is a real Penumbra game.