Exter, magnus, diduus, they all belong to the first declension and also the second declension.
Why these adjectives are so special?
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This is, perhaps surprisingly, how the vast majority of Latin adjectives work!
In general, there are two types of Latin adjectives: "first-second" and "third". These are all "first-second" adjectives, which take first-declension endings in the feminine, and second-declension endings everywhere else.
Note that this is based on the noun's gender, not its own declension: qvercus magna (f), nauta magnus (m), vulgus magnum (n).