You can embed external objects or Qlik Sense objects in Mashups.
Can also call different fields (Dim or measures) for an object. Not an expert in HTML/CSS/JS but you'll need these basic skills to do that.
Basic Mashup will be drag and drop of objects on a ready made template.
Learn/search more on 'How to build Masups in Qlik Sense.' There are many helpful videos on YouTube, specially by Brian Munz