@AEnigmatrix
As far as I understand it, the button layout is all the Japanese consonants and when you type words, you just swipe between the consonants while the autocorrect feature replaces the vowels as necessary.
Or you could tap several times on the button to switch between the vowels, like tadano mentioned with the flip phone (which had physical buttons)
I'm sure there's also an option to press and hold to directly switch to the wanted vowel, like you'd do in non English languages with special characters. Like if you held the button for "I" you can choose a special character based on that one. I.e. ï or ī