There are some colleges that say that they want letters of recommendation from core courses or one relates to your major, do u have to follow this??? or can you submit any?
If colleges say they need a certain teacher’s rec, you need to follow that. If you apply for biology, submit a science teacher. Don’t ignore a college’s instructions, it’s a surefire way to be rejected.
what if I am interested in public health? what teacher could i ask?
You want to ask a teacher who knows you well, both in and out of the classroom if possible. IMO, that’s more important than the subject, as long as it’s still a core subject. Your biology teacher might be a good to ask.
In general, it’s highly suggested to follow the instructions that a schools provides.
Bio would be a good choice, but any teacher who knows you well and can fully explain your capabilities would be a good choice. Even for Public Health, it would be better to have a history teacher that can specifically discuss your skills, motivations, etc, than a Bio teacher that can write “yeah, she was a student. She was pretty good”.