Yeah, put yourself in a puppies place: you've been taken away from the only things you know, you're now on your own with new humans in a new place, seeing, hearing and smelling new things - it's scary! It's best to edge them into things rather than do the accidental 'flooding' mentioned earlier, of having too much all at once. Start with short walks around quiet roads/traffic and build it up over time as you can see that the pup is confident enough (you should be helping at this time too of course). The same goes for meeting people and other dogs. Noisy children, walking sticks, sunglasses, hats etc all are things to be learned about and to learn to have no fear of, but that depends on those experiences. And yes, the same with dogs. They will have experienced plenty of rowdy behaviour from their own litter mates as they chew each other half to death most of the time, and they do need to meet and feel comfortable with other more full-on dogs, but probably best to start with calmer interactions and again build it up/vary it going forward, all with you as steward overseeing and encouraging and hopefully reinforcing the good things. The earlier you get this under way and the more you keep up with it, hopefully mostly positive, the better the result should be with regards to your own dog.
But I'll give you a quick example of how it can go wrong. I've seen a pup taken to a puppy socialisation class and all the other pups were running all over the place, chaos. The pup in question didn't like that and was kinda hounded by some of the pups to the point it was clearly not enjoying the experience at all. The idea is for them to learn that other dogs are safe and fun and not to be feared, but this pup was not being noticed for what it was showing - fear. The pup, unsurprisngly grew into an adult dog that wasn't comfortable with other dogs. Not just from this one experience but repeated experiences largely based on that intial one when it was flooded by too much stimulation all at once from loads of other pups and those overseeing not realising that it was nothing but bad experiences.