It can be done in either. There are 3 steps.
- Learn about greenscreen (keying) including lighting, filming, best formats for video (the way they do it in the weather segment) or masking and roto.
- learn about image editing including sizing images, color correcting, applying effects
- learn about controlling motion and editing duration of stills in a NLE or After Effects
When you have a few of these basics down and have designed your production you then load up the video and the stills, key out the background or mask around your talent to separate the talent from the background, add your photos to the time line and animate their duration and position.
This is extremely basic stuff that can be accomplished in a NLE or in AE. You can basically reproduce just about any effect you've ever seen in a movie in AE with the right planning and plug-ins. You can do just about anything you've ever seen in a news broadcast in any modern NLE.
Those newscast tutorials often use virtual sets. If you search on youtube for virtual set tutorials there are many that will help you understand how a virtual set newscast is created.