Let's say I got a degree in one field (business) and after I got that degree I decided that I wanted to get into the sciences so I applied for a bachelor's degree in science. What basis would university's usually use to determine if you get accepted? (i.e. do they use high school prerequisite marks, or your gpa from your first bachelor's)

Hi, Student,
After obtaining one Bachelors Degree, they will look at your transcripts of those courses... usually not looking at high school scores.
I got a Bachelors Degree in Math at 23 yrs old, then at 31 yrs old got a Masters Degree; and another Bachelors later than that.
Any of your first college courses could be used toward your second Bachelors, depending on the course requirements for your second degree, and also depending on the grade you got in any particular course.
I do wish you the best, and good luck.

Of course they are free to look at anything they wish, but normally only at your prior college work.

Even if the two fields are entirely unrelated?

It is not the field, it is the ability to learn and study they are looking at.

For example if you apply to law school, you may have a history major, an english major, even a engineering degree, none of which is related to law but it shows that they can follow though with their studies.

A college can look at many things, one will be other college grades, they may also look at perosnal recommendations if they require them. You can ask the college you are applying to what do they look at, they most liekly will tell you.