Do colleges let you know if you are going to get a scholarship after you get accepted or do you have to go to the college to apply for it? Lets say I get a perfect score in the SAT and all A’s with the GPA of 5.0. I get accepted to Harvard but just want to go to a college that will certainly give me a full ride, how will I know if I will get it?
For Harvard and the like, you will need to apply, get accepted, and then you will get your financial aid awards. Usually awards come either at the same time as the admissions decision or closely there after. You should have time to assess your awards before having to commit. You can however search for schools that offer full tuition to national merit finalists. https://college-kickstart.com/blog/item/colleges-with-great-scholarships-for-national-merit-finalists
Ivy League schools give need based financial aid, not merit aid. You should run the net price calculator (usually can be found in financial aid section or you can google it) in advance of applying to get an idea of what type of aid you might qualify for.
From another post you said you are a HS sophomore. You need to understand that Harvard and other Ivy level schools have acceptance rates under 10% (some closer to 5%) and must be considered long-shots for any unhooked (not a recruited athlete, not a child of a huge donor etc.) applicant as there are many more well qualified applicants than there are spots available.