I've never heard the distinction "apple head" Chihuahua before. My guess is it's someone's way of promoting a certain look in the breed- almost like it's a new variety. In my humble opinion, this is a marketing tool and sends off warning signals to me that that is not the way the breed should be and someone is breeding this style for a profit.
Be careful when buying a purebred dog from someone promoting a certain look and specializing in anything that deviates from the accepted standards for the breed.
In my humble opinion, getting a dog from someone who does health clearances on all the parents screening for things like bad knees, eyes, and allergies (very common in this breed) and making sure their lines are clear of defects is more important than the physical characteristics. Temperament is also incredibly important. After those criteria are met, then one can start looking at a litter and picking out the cute from the others.
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Chelle and the rest of the crew including, but not limited to Kita and Taiko (the shiba inu wrestle maniacs), Adi (reserved and dignified tabby cat), and all 28 reptiles