If you have a new iMac Pro on your desk, this article may be handy:
The memory in iMac Pro (2017) isn’t user-accessible. If the memory in your iMac Pro needs to be replaced, contact an Apple Store or Apple Authorized Service Provider. If you’d like to upgrade the memory in your iMac Pro, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific store or service provider that you contact offers memory upgrade services.