They are typically not taught in school, at least in the US but probably elsewhere too. I don’t even know of a commonly taken undergrad-level course that I’d expect to cover them.
They're often -- but not always -- touched on in advanced undergraduate classes, e.g. after real analysis and abstract algebra. Not every math undergraduate takes number theory these days.
Which classes are you thinking of?
My honors analysis class covered it. Although it the professor that taught the class worked in p-adic analysis so he snuck p-adics into the first quarter of the analysis sequence.