External Monitors Flicker when plugged into my Mac
More than likely, you need an active USB-C to HDMI adapter

The 2018 MacBook Pro has four (4) Thunderbolt 3 ports that utilize a USB Type C connector (this is where the confusion comes in).  Since it's Thunderbolt 3, it's actually  several signals in one port:


PCIe
DisplayPort
USB 3.1 Gen 2
Power Delivery


So, when you get one of those "USB-C to HDMI cables" you're actually getting a passive DisplayPort adapter and not an active adapter.  Basically, it comes down to the fact that DisplayPort and HDMI are very different signals - most notably, the clock sync signal isn't present.  So, to get a good reliable signal, you can do the following:


USB-C (DisplayPort) to DisplayPort - This is the best option because there's no conversion
USB-C (DisplayPort) Active HDMI Adapter.  If you must convert the signal, use an active adapter that actually re-creates the HDMI signaling.