I recently purchased a 2019 XT5 CPO. I believe the terms of the CPO may have varied over different model years and/or may be different for a Cadillac versus a Chevrolet/Buick/GMC, but I'm not sure.
For my 2019 XT5, the CPO warranty kicks in after the new vehicle "Bumper-to-Bumper" warranty expires (i.e., at 4 years from the original new vehicle In-Service Date) and adds two more years of a more limited warranty (and up to 100,000 miles). There is also a $50 deductible per repair visit and you have keep up the maintenance care and service requirements. The CPO warranty has a lot of exclusion - (I'm repeating here right from the front of the Cadillac CPO warranty booklet): "Warranty exclusions include, but are not limited to body and interior/exterior trim components, maintenance and wear items such as adjustments, batteries, brake pads and rotors, clutch discs, drive belts, engine tune-ups, hoses, tires, wiper blades, or any service or part required to be performed or replaced as recommended by the GM maintenance schedule". A little deeper in the booklet gets into a little more detail of non-covered items, such as most suspension components (shocks/struts, mounts, bearings, bushings, control arms, ball joints, etc.). And of course the warranty gets voided for damage due to accidents, misuse or alterations.
Flip side, a lot is covered for items that can get expensive to repair. Overall seems like pretty good/reasonable coverage and not a bad way to buy an automobile. Of course when buying a CPO vehicle, knowing that original In-Service Date is important, because that is when the clock was started for this net 6 years of coverage (4 bumper-to-bumper + 2 CPO). On my purchase, the vehicle had been in-service for only about 9 months.