From my experience with using Magic Lantern, and understanding of Canon cameras either do:
Canon 5D Mark IV/1Dc/1DX Mark II 4K:
1x1 (4K center crop)
Canon 5D Mark II/7D/T2i/70D and other 1080p Canon cameras released until 2014 (except the 5D Mark III):
3x3 (pixel binning horizontally, pixel skipping vertically, slight upscale) moire/aliasing/scaling artifacts
- Canon 5d Mark II: 1872x1053 effective upscaled to 1920x1080
- Canon 7D/T2i: 1728x972 effective upscaled to 1920x1080
- Canon 70D: 1824x1026 effective upscaled to 1920x1080
Canon 5D Mark III/IV/1DX Mark II/7D Mark II/80D 1080p:
3x3 pixel binning - 5d Mark III and most other Canon cameras since 2014 (better moire/aliasing and ISO performance than using pixel skipping)
- 5D Mark III (1920x1080 effective) sensor resolution chosen to avoid upscaling (5760/3=1920)
- 7D Mark II/1DX Mark II: 1824x1026 effective upscaled to 1920x1080
- 80D: 2000x1125 downscaled to 1080p (my guess)
- 5d Mark IV: 2240x1260 downscaled to 1080p (my guess)
Canon 720p Modes:
3x5 (pixel binning horizontally, pixel skipping vertically)
The Canon 5DS(r) is probably using 3x3 pixel binning:
2880x1620 resolution downscaled to 1080p (which happens to be the exact resolution of the Arri Alexa!)