Dell USB-C Monitor (U2520D) connection to an iMac (Retina 5K, 27-inch, 2017)
The only signal that comes out of your iMac is DisplayPort.  So, going from DisplayPort to DisplayPort is the best and most reliable connection as there’s no signal conversion.

USB-C is just a port type.  Thunderbolt is just a multiplexing of several signals:  PCIe, DisplayPort, USB, and power.  The cable that came with your monitor (USB-C) will support DisplayPort so, its perfect to use with your monitor; plug it into one of the two Thunderbolt 3 ports.

Converting to HDMI will undoubtably require an active adapter.  I don’t recommend it as it adds unneeded complexity because the signal must be converted; it’s inherently a point of failure.