Set up Keyboard
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: