To your 1st Q: We count how many people are in a community to know how much $ to give schools, housing, hospitals, etc.
If you don’t fill the Census, then planners believe people live in a community than reality &fund less.
(Immigration status will NOT be asked.)
If a state/city gets undercounted, then that community will literally get less members (votes) in Congress to pass key legislation on healthcare, climate, criminal justice priorities, etc.