Apple - What is this character and how could I type it?

That character is SCRIPT CAPITAL L (Unicode U+2112), usually seen in by documents typeset with LaTeX for Laplace transforms or Lagrangian mechanics.

\mathcal{L} results in: \mathcal{L}.

In macOS, the closest you'd get is searching for the character L in the Character Viewer (option+command+space) and looking under Related Characters for SCRIPT CAPITAL L.

It's included in the STIX set of fonts, which comes with MacOS since OS X Lion.

Copying the character results in: ℒ

Another option is to go to system preferences > keyboard > input sources and add "Unicode Hex Input" you can toggle the "show input sources in menu bar", or bind it to a keyboard shortcut. Then, when you want to type it, hold option while typing 2112, and the L will be typed. You could also configure it in Automator with the recording function and add that to a keyboard shortcut or on the menu bar.

Check out this font: Lucida Calligra