how do I insert symbols (heart <3) in mac mail?

You can also permanently add substitutions, so when you type <3 you'll get ❤ or ❤

by going into "System Preferences" > "Keyboard" > "Text" > and add them to: Replace..<3 with....❤

Hitting the control, command keys and space bar will bring up the character window where you'll find the emoji symbols and various others. You can drag any of them to your email message.