What is this fancy L with a curl on top, 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: .

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