One of the many great new features of QGIS 3.0 is its ability to view and work with multiple map canvases or map views. You can link the location and scales of these views together so that they are centered in the same place but showing different data at different scales. For instance you can make an overview map showing the location of the main data frame in a larger spatial context or a close-up showing detailed aerial photography.
Unfortunately, this feature is not very intuitive, nor is it well documented at the present time. I’m sure they are working on the documentation as I speak but I was able to eventually figure out how they work after seeing several pictures with multiple frames showing different data. This video is my attempt to explain the process in case you are as impatient as I am and want to use this feature now.
This video is also one lecture out of 76 in my course on QGIS 3.0 for GIS Professionals. This course has over 11 hours of video and is available now on Udemy.com for $20 You can read more about it here.