Surprise! The iMac Pro has Upgradable RAM and SSD - The Mac Observer
Upgrade your iMac 27'' 2017 RAM with MrMemory review - Wim Arys
How to Upgrade the Memory in the iMac Pro (late 2017) - YouTube
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Install memory in an iMac - Apple Support
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else