This talk by Marcin Wichary is — beyond both enthusiastic and outstanding — all about the complexity of UI design, typography, and the lengths his team at Figma has gone to make sure that doing something as simple as selecting a font from a dropdown does what you expect it to.
I’d recommend this talk even if fonts and typography don’t happen to be your cup of tea because Marcin’s stories about software development are always fascinating.
It was once famously said that the web is 95% typography. You can't be a web designer and ignore type! Frontend Masters has an in-depth course on web typography from Jason Pamental getting into things like responsive styles, variable fonts, font loading, and more.