You will have full responsibility for the AU's debt not them.
The lender cannot see your AU's credit because you will add them after the fact
An Authorized User is someone YOU allow to use your account. You add them and the credit card company gives them a card WITHOUT looking at their credit. In return and as a basis for that action, the credit card company will hold YOU responsible for any and all charges that the Authorized User makes.
The credit card company will bill YOU, not them. As far as I know (I have been an Authorized user for several years) the bill is never "split" and in fact is always sent to the account holder (you).
If you allowed someone to be an authorized user on your account, you have accepted full responsibility for any charges which that Authorized User approves or makes.
Be very clear...you not them is responsible and it is you not them whom the credit card company will seek payment from.
If I am wrong, it will be news to me.
Sometimes a lender will perform a soft pull, but never a hard pull. I have seen soft pulls with AmEx and with USAA when adding an AU.
The other person's credit history or lack thereof is generally irrelevant except for AmEx. If they have caused AmEx a financial loss in the past, AmEx will not allow them to be added as an AU. Navy Federal will sometimes do this, too.
Although you are fully responsible for any charges the AU makes, some banks hold AUs legally liable for their purchases in the event you do not pay. USAA does this as does, sometimes, AmEx.