Understanding action space in stable baselines
I guess I got answer to Q1. Sharing my understanding below:

PPO extends OnPolicyAlgorithm
PPO calls super(..,env,..), that is OnPolicyAlgorithm(..,env,..)
OnPolicyAlgorithm extends BaseAlgorithm
OnPolicyAlgorithm calls super(..,env,..), that is BaseAlgorithm(..,env,..)
BaseAlgorithm seems to obtain action_space from env

Still I am unable to get answer Q2.