I'd say you can trace while simplifying to try and understand the use of shapes and proportions and how the body and environment transforms, but you can also do that by referencing the image and working to understand the differences between the work and real life.
Out of the two styles you've highlighted, one is manga on the more realistic side of what people generally think when they hear "manga" and JC Leyendecker is straight up realism from things like old sports illustrated magazines and adverts from before photography was commonly used for those so it seems like you're aiming for slightly simplified realism.
Also from the looks of it, both are traditional art (or possibly digital art imitating old traditional art looks in the case of Nishimura). Leyendecker is obviously using paint, either acrylics or oils for their work, and Nishimura's line and colour technique is very similar to traditional ink lineart with watercolour and alcohol marker colourwork. It might be a good idea to test out some traditional art methods if you haven't already, some things are just easier to obtain with traditional mediums.