How do i compose utf-8 characters from the keyboard (osx 10.11.4)?

Here is how to enable the unicode keyboard:

Go to System Preferences and click Keyboard:

Click the + button in the lower-left corner:

Search for unicode:

Click Unicode Hex Input and click Add:

The Unicode Hex Input is now available from the system toolbar:

After selecting Unicode Hex Input, hold down option while you type in the unicode for the desired character. For example, option-00a0 is non-breaking space.

OS X doesn't use utf-8 for input. Normally you would use the Character Viewer (Edit > Emoji and Symbols) to input characters which are not already included in the keyboard layout via option and option plus shift. It has an option to show all of Unicode.

If you really want to input special characters via numbers, you can go to system prefs/keyboard/input sources and activate the Unicode Hex keyboard layout. With that you can input any character by holding down the option key and typing its 4-digit (or 8-digit for those beyond the BMP) Unicode hex number. e28099 is 2019 in hex. It's the curly apostrophe which can also be made directly from the US keyboard via option shift ]