Question about Lambda

Hi, i am very confuse about the syntax of Lambda, please refer attached.
I am doing a Textjoin by criteria thing. (its own exercies)
My formula in D2 work, but i Memories it rather than Understand it.
I do not understand why is it like that. I written down my question in the excel in yellow.

Also, after i textjoin, i try to textsplit back to original using formula, but i do not know how.


Anyway can help shine some light?