This is really a fantastic resource - it doesn't hide too much away behind "helper" libraries so you understand where everything is coming from. It's the only beginner resource I've found that covers drawing multiple things: that was always my stumbling block with WebGL!
Shameless self plug: You should check out my Youtube channel. I have a playlist that I cover Webgl (not 2) https://www.youtube.com/watch?v=oDiSqQT_szo&list=PLPqKsyEGhU.... I've done 108 WebGL screencasts, everything is built up slowly over time from scratch (so nothing is hidden away), and covers a lot for beginners to include covering drawing multiple things. Cheers!
When you've exhausted this and are looking for next steps to try, I recommend this youtube series by Sketchpunk Labs: https://www.youtube.com/watch?v=LtFujAtKM5I&list=PLMinhigDWz...