The quickest way to type a heart is to copy and paste one, but there are keyboard shortcuts too.
Windows
Hold Alt and type 3 on the numeric keypad, then release Alt to get ♥.
Mac
Open the Character Viewer with Control + Command + Space, search “heart”, and double-click the one you want.
Phones
Copy a heart from a symbol page, or add a text replacement so a short code expands to ♥.
Facts to verify: the Windows Alt+3 shortcut and the Mac Character Viewer shortcut on current OS versions.