Would a person with "good" credit get the same interest rate on the same loan as someone with "bad" credit, and why?

No. A person’s loan and repayment history is translated into a credit score. Credit scores are calculated by several different companies but usually range from 300–850 points. The higher your credit score, the better your credit is for loans.
Lenders are taking a risk when they give you a loan. Your credit score tells them if you have repaid your prior loans or if you have struggled with late payments. The better your credit score (credit repaying history), the less risk the company is taking on you paying the money back. They can therefore give you the loan with a lower interest rate. On the other hand, if your credit score is poor, a lender will charge a greater interest rate to cover the greater risk in the event that you default on the loan or declare bankruptcy. Most lenders have a minimum credit score requirement and will not provide financing to individuals who fall below that minimum score.