About Japanese (Kana) letters
Japanese mixes hiragana (the first grid), katakana (the second) and kanji. Kana are typed by romaji: ka gives か, and the space bar converts to kanji.
Every OS ships a Japanese IME: Microsoft IME, macOS Japanese - Romaji, and the Japanese Romaji or Kana keyboards on phones.
Dakuten ゛ and handakuten ゜ modify consonants (か → が, は → ぱ); Unicode has precomposed forms for all of them.
Japanese (Kana) letters: names and codes
Every letter with its name and Unicode code point. None of these have classic Alt codes; in Word type the hex code and press Alt+X, or use the HTML code on web pages. Click a letter to copy it.