PvT 2base observer/Colossus opening

Harstem's "Supply Challenge" guide (2020-09)

Here's the build order at SpawningTool.

You may still want to consult Harstem's previous guide on this playstyle for a broader overview of possible reactions and responses, just below.

Harstem's video guide, from October 2019

Gemini's BOTW, from December 2019

SpawningTool, and the attached Reddit thread.