How do people get accepted to good colleges with low SAT scores or low GPA's?

Depends. Some schools may care a lot about GPA, but not care about SAT scores. Some students may have extenuating circumstances that caused them to have a low GPA. Some students may have had a poor GPA, but significantly improved. Some students have great letters of recommendation. Some students have connections. Some colleges bend the scores/GPA they report (for example, they may post the average weighted GPA).