If I graduate community college with an associate degree and apply for transfer to Ivy Leagues and T20 universities, even if they require high school transcript the HS GPA doesn't matter at all right?
This would be an incorrect assumption.
Many Ivys like Harvard, Yale, and Columbia require an HS transcript as part of your transfer requirements. What you did in HS does matter to them.
It would matter less per se if you transferred from Santa Monica Community College into one of the UC schools. In that case, the UC schools are more interested in your CC GPA not your HS GPA.
To keep this community safe and supportive:
if you graduate Community College with a 4.0 GPA, then something like a 2.0 GPA in HS won't impact your admission chances right?