Apple - How to type dʒ symbol (IPA) on Mac?

With ABC (formerly US) Extended, ʒ is made by typing option-shift ; and then z.

In general, to see which key does what, use Keyboard Viewer.

More info about ABC Extended can be found here. And here.

If you know the Unicode name of the character, you can use ⌘ - ^ - Space to bring up a character search. This character is called Ezh:

Usually this is used for Emoji, but as you can see, it works for any character.