(1) Why connect 1V8 LDO input to 3V3 LDO output?
(2) How to select MOSFET as switches in hot swap (PSU / HDMI) applications
Suggest to first google/wiki LDO to get a rough idea, then References (1) to
(3), especially (3) to understand your first question.
/ to continue, ...
MOSFET as switching device in PSU/...