Follow-up questions
For Foundry v10+ here is the essential info for module integration as I gather you might want to load a font or two for whatever your module does:
https://foundryvtt.com/article/v10-font-config/
When looking at the core Foundry client code check out
https://foundryvtt.com/article/v10-font-config/
When looking at the core Foundry client code check out
FontConfig as this manages the process for Foundry.