For the record: Cats are great moms. They take diligent care of their babies.
If you come across kittens of any age outdoors, the odds are overwhelmingly against them having been “abandoned” by their mother. That just generally doesn’t happen. Occasionally an accident will befall the mama cat, leaving the kittens orphaned but that, too, is rare.
Mothers will leave the kittens to search for food. They will also relocate their kittens if they find a better spot to raise them, or if they feel threatened.
Finding one or more kittens alone on your doorstep (or next to your car, or in your yard, or just about anywhere) does not mean the mama cat has “abandoned” or “rejected” them. It likely means you interrupted her in the process of moving them.
If you find kittens, please stop. If they are not in any obvious danger, simply leave the area. Don’t hide nearby because the mother cat can still smell you and sense your presence. Check back in a few hours to see if the kittens have been moved and/or the mother has returned.
Keep in mind that kittens have the best chance of survival with their mother. If they look healthy and well cared for, leave them for the mother cat to raise.