On iOS

Open your system settings app “Settings” -> “General” -> “Keyboard” -> “Keyboards”:

Click “Add New Keyboard…” to add a Japanese keyboard: either Japanese– Romaji (recommended) or Japanese – Kana.

On Android

Google Pixel: System settings-> “Languages and Input “ -> “Languages” -> search for Japanese

Samsung Galaxy S21: System settings-> “General management” -> “Keyboard” -> “Samsung keyboard settings “ -> “Languages” -> “Manage input languages” -> search for Japanese

Screenshots of a Google Pixel: