Affinity Designer: Add a New Font
Affinity Designer accesses the fonts installed in your system. So to use a new font in Affinity Designer, you have to install it in your system.
It comes as a ZIP folder consisting of an OTF file (Open Type Format), which is supported by macOS, and an .RTF file outlining its usage rights.
Double-click on the OTF file.
A font preview window opens up.
Click on the "Install Font" button.
After successful installation, if you click on the Text Tool and expand the Font dropdown field in the toolbar, you will find the Anurati font in the list.
Here is Rocket Raccoon, typed with the Anurati font we just installed.
The whole process is outlined in the official Apple support site here.