You can connect many different types of Thunderbolt and USB-C devices to the ‘downstream’ Thunderbolt/USB-C ports on the front of the dock.  The type of functionality provided by the port of the dock is ultimately determined by the capabilities of the host computer.

To put that another way, if the host computer has Thunderbolt 4 technology built-in, then the downstream port will function as a Thunderbolt 4 port. 

If the host computer has Thunderbolt 3 technology built-in, then the downstream port will function as a Thunderbolt 3 port.

If the host computer has USB-C technology built-in, then the downstream port will function as a USB-C port.

An important note with is that external GPU's (eGPU's) are not supported on macOS, so they will not function when connected to the downstream Thunderbolt port on the TBT-UDM