Who could I ask to provide recommendations for my US university application?
I would say mentor figures who know you in some way would be the best candidates to write your recommendations. For example, I had my Creative Arts Program mentor and my Humanities and Social Sciences Research Program mentor write my recommendations for my Ivy league applications. A teacher who knows you well or an elder in your community would also be a good choice.
When you are filling out the common application, schools will specify what types of recommendations they’d like. Some schools just required teachers, and they’ll know this because you’ll need to submit their teacher email. I would suggest getting at least 3 (some schools only require 2, but you can and should submit 3 if you’re able to and if they’re from good teachers), two from teachers you’ve known for a long time, and one from a teacher with a high position, like Head of Science or a Principal or Vice if they know you well. They’re usually asked to answer very specific questions about your classroom or learning style and behavior, so keep that in mind that they can’t write their own nice sweeping gestures - they should really know you as a person.
You can then often also submit “other recommenders” which are letter from people who aren’t teacher - can be coaches, mentors, managers. Often these “others” will know you perhaps better than a teacher because they might have coached you for eight years, so they’ll know a lot about your character and nature.
It’s important that recommendation letters cover and showcase all parts of your being - your academics and your spirit. I would provide your recommenders with a couple of anecdotes from your time in their class/sportsteam/company so they have something tangible to reference.
For grad schools:
This varies from school to school and faculty to faculty. So you’ll need to do a lot of research on what the school culture is and what type of student the course is looking for, and gear your recommendation letters to that. E.g. if you are applying for an engineering Masters program at a school like Stanford which fosters entrepreneurship and innovation, you’ll want recommenders to showcase your innovative spirit and when you’ve thrived in a project.