Running Worlds is easy. Click World images to run them. Try running the Editor's Choice Worlds or Random Worlds.
There are currently 7,569 Worlds on this site.
There are introductory pages:
library is a little more intimidating for a beginner coder.
But it is well suited to non-beginners.
It is designed for advanced graphics.
There is currently no Three.js teaching course on this site.
On the other hand, almost all the advanced Worlds on this site are Three.js.
The P5 library is probably easier to get into if you are a beginner coder. But it has some features that may annoy non-beginners. It seems to be harder to do more advanced graphics with it. There are a number of P5 teaching courses on this site.