I learned: MacOS has a Unicode Hex Input keyboard

A while ago, I learned that MacOS has had a Unicode Hex Input keyboard since ages.

It is not installed by default, so you have to manually add it:

1. Start the System Preferences.app
2. Open the Keyboard icon
3. Choose the Input Sources tab
4. Click the plus (+) icon
5. Search for Unicode or Hex to get so Unicode Hex Input is the only entry in the list
6. Click the Add  button
7. Choose the Keyboard tab
8. Enable Show keyboard and emoji viewers in menu bar

Now in the menu bar, you can select the Unicode Hex Input.

After that, when holding the Option key, any 4-digit Unicode sequence will get you a Unicode character.

From [Wayback/Archive] How to SIMPLY insert thin spaces using a … – Apple Community:

enable Hex Unicode Input, and then while holding down the option key, press 2+0+0+9, the unicode number for thin space. You will see it insert. Unicode and HTML codes for thin space here.

To enable Unicode Hex Input:

System Preferences > Language & Text > Input Sources tab > ✓ Unicode Hex Input

From this, I found it back in Unicode input: Hexadecimal input in MacOS – Wikipedia

Hex input of Unicode must be enabled. In Mac OS 8.5 and later, one can choose the Unicode Hex Input keyboard layout; in OS X (10.10) Yosemite, this can be added in Keyboard → Input Sources.

Holding down ⌥ Option, one types the four-digit hexadecimal Unicode code point and the equivalent character appears; one can then release the ⌥ Option key.^[9] Characters outside of the BMP (the Basic Multilingual Plane) exceed the four-digit limit of the Unicode hex input mechanism but can be entered by using surrogate pairs: holding down the ⌥ Option key while entering the first surrogate, the +, the second surrogate, then releasing the Option key.

–jeroen