The 6D has a very basic AF system. The 5Ds has one of the most complex and configurable AF systems of any camera on the market. It is more or less the same system used in the 1D X, the 5D Mark III, the 7D Mark II, the 1D X Mark II, and the 5D Mark IV.

As with any sophisticated tool, the more options you have the more you need to understand what each option does and how selecting a particular option interacts with all of the other selected options to affect overall system performance.

You're not going to figure out the 5Ds AF system in five minutes. Just playing with it, you're not going to figure it out in five hours or five days, either.

You need to read the Manual carefully. I would suggest reading all of the sections that apply to the AF system and then going back and going through it again section-by-section as you experiment with your camera.

Beyond the EOS 5Ds/EOS 5Ds R Instruction Manual there are also several AF guides published by Canon. The EOS 5D MArk III AF Setting Guidebook is a good introduction and overview to the AF system in the 5D Mark III which is nearly identical to the AF system in the 5Ds. The EOS 1D X Mark II AF Setting Guidebook is a much more in-depth look at a similar AF system, although your EOS 5Ds does not include all of the features of the 1D X Mark II's AF system, much of the material in the 1D X Mark II's guidebook would be beneficial to you. Particularly, the parts about when different AF modes and use cases are most effective should be required reading for any owner of a 5D Mark III, 5Ds/5Ds R, 7D Mark II, or 5D Mark IV as well as the 1D X Mark II.