The engine is the viewport
What you see while editing is the image the player gets: same lights, same post-processing, same pixels.
Visual editor · in the works
The editor is on the way
A visual editor where the engine itself is the viewport: you place objects while looking at the real game, not an approximation. It is not downloadable yet.
The editor video goes here
A scene is a text file
Everything you do is saved as readable JSON next to your code. It goes into git, it reviews as a diff, and it does not need the editor to be open.
Play without exporting
Hit Play and the game runs inside the editor through the same code the exported build uses. What you see here is what ships.
In the meantime, the engine already works
The editor comes later; the library works today and you write it in ordinary TypeScript. The tutorial builds a whole game on this very page, nothing to install.