You have to go to each school’s website to see how they handle outside scholarships. Meet full need schools require students to report ALL outside scholarships, whether they were paid out to the school or applicant. Schools that don’t meet full need typically don’t engage in scholarship displacement.
Even though you didn’t apply to Northwestern, they have a good primer on external scholarships with several examples, see here. From NU’s website “Federal regulations require that these scholarships be evaluated and incorporated into a student’s financial aid award, and adjustments may need to be made to existing aid”.
That’s not to say students can’t still benefit from outside scholarships, as one example shows these scholarships are typically used first to eliminate/reduce any work-study and/or student loans (both “self-help” items).
Talk to your HS GC, they should have a list of local scholarships. Good luck.