How to make a video presentation with a video of the slideshow and a video of me speaking in the corner?
Preferable depends... But possibilities are for example: MacOS offers the App iMovie which is available with no further cost. For more professional video editing you can use e.g. Adobe Premiere or Final Cut, but then you need to pay and you need much more effort doing research to get familiar with it. To sync two videos and the iMovie App will be fine.