Why a step-in harness suits a puppy
A puppy who has never worn anything before will usually object to something going over their head. Our step-in design does not. You lay it flat, they step in, and the neck and chest clip closed. It is a calmer first experience, and calmer first experiences are what stop a dog hating their gear for the next decade.
Both the neck and the chest adjust, so there is real growing room built in. Most pups get several months out of an XS before moving up.
For walking we would put the lead on the harness rather than the collar. The collar is for their tag and their name.