A credit cardholder adds another person as an authorized user on their credit card account. Then, the entire history of that credit card account appears on the authorized user's credit report and included in their credit score. None of the cardholder's other cards or loans will appear on the authorized user's credit history, just the one card.
Being an authorized user on a credit card account that has a positive payment history would boost your score (granted the credit scoring calculation includes authorized user accounts in credit scores). On the other hand, late payments and high credit card balances for the authorized user account, could lower your score depending on the other information on your credit report.
Note: The latest mortgage credit reports use the Fico version 8 credit scoring model that only factors family and household members authorized user accounts into the scoring model.