Under the Input Sources tab,
1) checkmark ‘Show Input menu in menu bar’;
2) click on the + button to activate the new
Input Unicode Hex keyboard layout
(see next screen shot)

The dropdown menu allows you to activate a new
keyboard layout (in addition to the one you
already had active):
3) start keying the name of the new keyboard layout
you wish to activate (Unicode Hex Input);
4) when ‘Unicode Hex Input’ appears selected
in the right-hand pane, click ‘Add’

In your menu bar, near the right, you now see
that you have two active keyboard layouts:
one is your primary keyboard layout (in this case
‘ABC’, a Latin-script keyboard associated
with the English language),
and the other is Unicode Hex Input,
which you can now select and use.

Note also that you now have access to two new items:
‘Show Emoji & Symbols’, a bit of a misnomer since it
in fact gives you access to all of Unicode;
and ‘Show Keyboard Viewer’, which is self-explanatory.