VESA standards for common display arms:
Most displays are divided into three VESA standards: VESA MIS-D, VESA MIS-E, and VESA MIS-F.

12-24 inch (up to 14kg) monitors typically have VESA-MIS-D ports, with dimensions of 75x75mm or 100x100mm;
25-30 inch (up to 23kg) monitors are usually VESA MIS-E ports, with a size of 200x100mm;
Monitors or large televisions over 30 inches usually use the VESA MIS-F standard, but general users do not need to use such a large size. VESA size: spacing of 200 millimeters (such as 400×200, 600×400 millimeters)
Previously, most Apple monitors did not comply with VESA standards. To install an Apple monitor without VESA holes onto a VESA bracket, you may need an adapter bracket as follows. This adapter bracket is specifically designed to solve the problem of Apple monitors without VESA holes. It can easily fix the monitor on the VESA bracket, providing users with more installation options and flexibility.

VESA Adapter Kit for iMac Pro
The VESA installation adapter kit provides everything you need to replace the iMac Pro bracket with an installation bracket. After connecting the VESA installation adapter, your iMac Pro meets the VESA FDMI (MIS-D, 100, C) version specifications. This adapter supports iMac Pro and cannot be used with any other iMac or monitor. The adapter can only be used with VESA brackets that can support the weight of iMac Pro.

VESA Adapter Kit for iMac Pro Display XDR

Apple’s iMac Pro Display XDR now offers a choice of VESA interface accessories, allowing you to modify your XDR monitor to support VESA Plate connection to the monitor bracket. You need to spend $200 to purchase Apple’s VESA installation adapter, which can connect the iMac Pro Display XDR to any VESA compatible wall mounted, desktop, or articulated arm, creating the ideal workspace.

Other iMacs
Please check below article: Can I mount an iMac on a monitor arm?