Extensions: Application Customizer - Loading JS Files in <HEAD> Tags
The current architecture of the SPFx Extensions allow for us to populate "Placeholders", such as PageHead and PageFooter with content.
If we need to load JS files though, be them stored within SP or externally, we would need to add external references into the <HEAD> tag of the page.
We can use the current placeholders to dynamically update the <HEAD> tag, but this seems less then optimal.
Classic UCA's allowed for multiple JS references, as well as the sequence in which they were loaded. (Ex: Load jQuery prior to loading a Utility JS file that is dependent on jQuery.)
Replicating this functionality for SPFx Extensions would allow better inclusion of JS functions needed site wide.
Hope you find that useful.