There are countless things that a puppy is socialized to without owners doing any purposeful training. These are elements that a puppy comes into contact with or things that a pup interacts with throughout the course of his days. However, dogs that have been neglected may have been denied this natural socialization.
This includes everything from walking on different surfaces (grass, pavement, carpeting, etc.), going up and down stairs, hearing and seeing household items like the vacuum, coffee grinder, or dishwasher, and even sleeping on a dog bed.
Even assuming that a Chihuahua was raised with natural and expected socialization, many do still need purposeful training to become accustomed to people outside of the household and to other dogs.
In regard to people, it is normal for Chihuahuas to be very attached to their owners but a bit standoffish and wary of others. They may bark at or even lunge at neighbors or people that come close when out and about. Though, others may cower down, shake, or otherwise
display fear
during encounters.
In regard to dogs, it is natural for a small Chihuahua to be apprehensive of a much larger dog, but since larger dogs can pose a legitimate danger to toy breeds, this is not necessarily a bad thing. However, for the sake of sanity when walking your little guy or gal, it is a reasonable goal to be able to walk your Chihuahua without wild barking fits and lunges at other dogs.