I don't understand this. How does tromping through the woods and seeing many ivy-looking plants teach you anything, unless you have a way to know definitely which are poison ivy and which are not?
The sapling form is not quite the same as the vine form, which is not quite the same as the very thick hairy vine that it eventually transforms into. But once you know one, you can start to recognize the others due to at least some parts of the plant matching what you already know. And then the other parts of the plant, which you hadn’t previously recognized as being how poison ivy can look, are added to your personal visual memory, too.
I don’t know, it’s hard to explain. It’s like if you saw a chihuahua and a Great Dane, you might not recognize them as being the same species. But if you see enough dogs in between, eventually you’ll see that they have something in common.
If it's showing as a vine you just have to categorize the attributes of the other vines. Where I'm from there are only 3 or 4 common vines. Grape, Virginia Creeper and Poison ivy. Also there are non-natives: kudzu and english ivy, but they exist in more specific contexts.
Your brain learns to categorize the attributes of these vines into different buckets and it sticks with you. Grape for example just kinda hangs around in the air with thick woody rope-like vines. Virginia creeper looks pretty similar to poison ivy, but really desperately hangs on to the trunk of trees. Poison ivy hangs on to the tree too buy tries to branch dramatically towards light often with parabolic woody branches. Mature Virginia creeper is hairy like poison ivy but the hairs are lighter, thicker and shorter than poison ivy. Your brain categorizes all these differences even if it doesn't know the name.