Overlays are available only in the 'Develop Persona' of Affinity Photo. Layers are a way how it works in the 'Photo Persona'. An overlay is more like a mask while a layer can be an adjustment, a pixel layer or even a live effect layer.
Quote from the help file:
Overlays are elements within Develop Persona which are placed on top of an image. Any adjustment applied to an overlay affects the image below.
However, overlays can have a varying level of visibility, which varies the impact its adjustments have on the underlying image. In this respect, Overlays in Develop Persona act in a similar way to masks in Photo Persona.
The most recent version of Canon Digital Photo Professional (DPP) is 4.7.20 and it will open your 6D Mark II cr2 raw files fine.
You can download DPP here: https://www.canon.co.uk/support/consumer_products/products/cameras/digital_slr/eos-6d-mark-ii.aspx?type=software&softwaredetailid=tcm:14-1622062
Then if you click on the 'Manuals' section, you can download the DPP manual: https://www.canon.co.uk/support/consumer_products/products/cameras/digital_slr/eos-6d-mark-ii.aspx?type=manuals&language=
