If you mean static files (for example bootstrap's css and js files), you often put these into the /public folder - I think they are served from here by default, but it might have been changed.
For further reference you can see https://github.com/codeparty/derby-starter/blob/master/lib/server.js#L47 where a folder is specified for serving static files. Instead of expressApp.use(express.static(options.static))
you could probably use something like expressApp.use(express.static(__dirname + '/yourstaticfolder'))
instead.
The ui folder is basically an example of how a component can be included and used in your application.