Subject of "introduced" ?

Hi~ I'm not sure about the subjects of "introduced" in the red part and "has" in the blue part.

In both cases, there seems to be two possible answers grammatically correct but

I can't figure out which one is reasonable. Please check this writing and let me know your opinion.

Thank you~!

Memories that can be recalled quickly are better able to link to incoming information to aid in retention and comprehension. When a well-developed body of implicit knowledge exists, it begins to generate understanding and even create abstract concepts (when introduced or independently obtained). A brain possessing a wealth of related facts that has an understanding of a range of related concepts begins to produce more advanced thoughts, often referred to as higher-level thinking.