To start with it turns out the system is very similar to the older 2015 model so a full teardown won't offer much from a repair perspective.
Yes, understanding the internals can be useful for future upgrading! So to help you see the insides, take a look at this video: Unboxing and Teardown of the 2017 27" iMac retina 5k OWC did.
You may want to also review the EveryMac listings: 27" iMac's 5K (Mid-2017)
As for upgrading the HD with either your own HDD or SSD drive you will need the OWC in-line sensor: OWC In-line Digital Thermal Sensor for HD Upgrade (HDD or SSD) for 27" iMacs 2012 and Later
The CPU does appear to be upgradable (socketed)
Not sure I understand what you mean by 'Phases' are you asking how many cores or threads the CPU offers? The EveryMac listings should answer that based on what model you are thinking of as well as the GPU and VRAM each uses.
To understand the CPU's details I would recommend you visit Intel's ARK site. Here is a breakdown of the major three:
And lastly reference the older iMac Intel 27" Retina 5K Display (Late 2014 & 2015) guides.