I found a few open-source small bipedal robots on the internet. There are actually many open-source robots available:

[1]. https://robotics.umich.edu/news/2024/a-smaller-simpler-biped...

https://github.com/daviddoo02/Legolas-an-open-source-biped

[2].https://github.com/MuShibo/Micro-Wheeled_leg-Robot

[3].https://github.com/Yuexuan9/Tinker

Quadruped:

[1].https://github.com/stanfordroboticsclub/StanfordQuadruped

[2].https://open-dynamic-robot-initiative.github.io/