my question is mentions generic structure of announcement and explain in detail

announcement is used for announce something in general for many people.

example :
annoumcement
the student who have the final exam conflict schedule, please go to meet the staff at the office of the registrar, engineering building 3, 1st floor. the deadline is on april 26, before 5 p.m

in the text above, we can conclude that announcement is made for inform something to many ppl.