For bio or pre-med, you would apply to the life science program under the Faculty of Arts and Science. To do this , you would use a OUAC 105 application. You can switch your major at any time. However, you would be unable to transfer to engineering and unlikely to transfer into commerce as those are different faculties. At U of T, you’d be required to do 2 majors or one specialist. There are a couple of other combinations including minors that I don’t recall right now.
The life science program would require that you have taken calculus. Your admission would depend on your grades in calculus, English and your next 4 highest courses. You should submit your SAT or ACT scores as well as any AP or SAT II scores that are good. There is no interview or essays required.
There are 7 colleges. http://www.artsci.utoronto.ca/futurestudents/colleges/index_html A couple of the colleges, Trinity and Victoria, will require you to write essays to be admitted. They are also 2 of the colleges that will not admit you unless you rank them as first on your college selection form.
The largest lecture hall on campus seats 1300. You might have a first year Bio lecture there, but that can be avoided if you wish. When you select your courses, you can see how many seats each section has, so you can select one with 1300 seats, or you can select one with 300. The upper year courses generally have much fewer seats. The median for upper year courses is probably around 60 seats. First year courses, like biology, will also have weekly tutorials which are led by a TA, and those have a maximum of 24 seats. So, I would say that U of T is like the big state schools in the US.
Yes, you can get into top US grad schools from U of T.
Since you are in Michigan, I would recommend that you attend U of T’s fall campus day which is held on a Saturday in September or October. You can tour the different colleges and there a bunch of sessions about various topics of interest to students who are applying. http://www.electronicinfo.ca/universities/toronto-st-george/events/79
I recommend applying as early as possible eg around November.
Good luck!