Creating UI for Figma Plugin

I’m trying to make an UI for my friend’s figma plugin, but I can’t understand certain things.
Is ui.html the only html file I can use in plugin?
How do I use custom fonts and import images into the html?
Can I somehow make a separate css file and link it to ui.html?

No, you can have a map of html files and then call the desired html. Read more here: __uiFiles__.