Here's the path I would suggest to learn Mashups.
1. Read through the doc in the developer help Mashups ‒ Qlik Sense
2. Try out the Mashup Editor in the Dev-Hub where you can use some pre-built templates. This could be augmented with the Qlik Training class "Create Mashups with Qlik Sense"
3. If you want to go deeper, take a class such as
or the San Francisco class I am offering in September