I have adopted 2 dogs in the past that were already 6 months old and both were abandoned by their first owners. The socialization period is when the character of the dog is shaped the most, but that doesn't mean the dog simply stops learning and changing behavior after that time.
In my personal experience, the following events shape the dog the most during socialization and are hard to change after that:
- Contact to other dogs. If the puppy didn't have many opportunities to interact with many other dogs, it may become insecure or aggressive towards dogs later in life. If the puppy did interact with many different dogs, it probably learned how to communicates with and act around dogs and there probably won't be much aggression when meeting other dogs.
- Contact to other humans. If the puppy had some traumatic experiences with humans early in life, it may become insecure or aggressive towards other humans or avoid humans as much as possible. It's very much possible for the dog to build a positive relationship with its human family, but strangers may always be a problem. OTOH, if the puppy had contact to many different people as a puppy, it probably becomes more social and welcoming towards strangers later in life.
- Any traumatic event. One example: our current dog was abandoned in a forest and was caught by either some rangers or soldiers or someone with green clothing. She's still afraid of anyone wearing green clothing because of that, even if she knows the person and interacts with them when they wear any other color.
- Climbing stairs. Dogs need to learn how to climb and descend stairs and if they didn't learn that as a puppy, some never learn it.
Other things like riding cars, bathing or clipping nails might be frightening at first for a dog, but in general they can be taught to accept these events even long after the socialization. It may take longer, but it's not impossible.
By the way, the moment you bring your dog home, you start a short almost-socialization phase. You cannot completely change the character and behavior of your dog anymore, but you can teach them how they are supposed to act, for example whether they are allowed on the sofa, on the table, on the bed etc. This phase usually lasts for 2 weeks, but can last longer for insecure or anxious dogs.