If Im in an Accident in Pennsylvania in Someone Elses Car, Which Insurance Pays My Bills?

Which insurance company is supposed to pay your bills depends on the type of coverage (i.e., wage loss, medical, bodily injury) and the amounts of coverage on each policy. There is a "priority" as to which insurer must pay first.
If you were driving or a passenger in someone else's car and were injured in a car accident, there are layers of "priority" as to which insurance company pays your bills

Medical Bills Wage Loss Coverage

For Medical bills, if you own a car, your medical bills are covered under your own household auto insurance policy. Up to the limits of your coverage. Your own policy 'follows' you for medical. If you don't own a car, then your medicals would be covered by the insurance of any household auto insurance for any resident relative. This means that if you live with a family member (it must be a 'blood relative'), their auto insurance will cover your medical bills. Even if you are in someone else's car. It will not raise their rates - that is part of what they paid insurance premiums to be covered for. If you don't own a car and don't live with a resident relative who owns a car, then your medical bills will be covered by the "host" vehicle (the vehicle you were traveling in at the time of the accident).

Wage loss is the same as medical. Both are called "first party benefits" or "no fault benefits" in Pennsylvania. However, wage loss is not required. So if neither your household auto insurance nor the host vehicle have wage loss (or not enough wage loss) coverage, then the at-fault insurer must pay your lost wages. But, the at-fault insurer likely won't pay those until the end of your case.

Bodily Injury

The at-fault driver's liabilty insurance will be the first "layer" of insurance to cover your pain and suffering (bodily injury) damages. If your bodily injury damages exceed the coverage on the at-fault driver's policy, you will be eligible for "underinsured" motorist coverage. The first layer of underinsured coverage will be on the "host" vehicle. If that host vehicle doesn't have underinsured coverage (you are not required to have underinsured coverage in PA), or if the host vehicle's underinsured coverage is not enough, you can then apply for underinsured coverage through your own auto insurance policy.