https://maps.mundialis.de/ is for anyone looking to explore data from the Sentntal satellite. But what I think is more interesting is the topographic layer built into the web app which allows you to interactively zoom around the planet.
There is a complete description as to where the data is collected from and how it was processed here
At the bottom of the data source and processing description, you will see this URL to a publicly available web mapping service (WMS). You can read more about what a WMS is here
By adding this link to QGIS you can add this beautiful layer of relief data to your maps!
Here is the URL for the WMS topographic service https://ows.mundialis.de/services/service?SERVICE=WMS&REQUEST=GetCapabilities
And this short video shows you how to add it as a layer in QGIS
I sure the web map application is amazing if you really know what you are looking for in terms of Sentinel data. But personally, I think the ability to access a topographic layer for the entire planet via and open WMS is the most exciting thing about this web application.