Shown below is the OWC 128GB memory kit installed in the 2019 iMac 5K. Installation took all of two minutes (well, maybe 3 or 4 including unplugging, positioning, installing, plugging back in). Pop the back cover, remove existing modules, install the new modules. See the memory install video for 2019 iMac 5K.
The smart move for users looking for 32GB or more memory is to order this 2019 iMac 5K with 8GB memory, then add 32GB or 64GB or 96GB or 128GB at about half the price Apple charges (and Apple does not offer 128GB). In other words, get 128GB for the price Apple charges for 64GB—a no-brainer.