The USB consortium specifies using HDMI 1.4b and DisplayPort 1.3 (and MHL 3.0) on the Type-C port. So non-Thunderbolt machines have these specs as a maximum.
Thunderbolt 3 can also pass video signals, and Intel specifies different versions of the protocols: HDMI 2.0 and DisplayPort 1.2. Again, these are the maximums.
So if you have a USB Type-C monitor (not Thunderbolt) or a native USB Type-C to video cable, you're limited to HDMI 1.4b (1x4K at 30 Hz) but instead you might be able to use DisplayPort 1.3 (1x 4K at 120 Hz). If your video card, connector, cable, and display supports it, of course.
If you have a Thunderbolt-native monitor, you might be able to do 2x DisplayPort 1.2 (4K at 75 Hz or 5K at 30 Hz) or 2x HDMI 2.0 (4K at 60 Hz and much more). If your video card, cable, and the Thunderbolt controller support it.
Essentially, you can pass video directly over the port (in USB Alternate Mode) or over Thunderbolt. Then there are external video adapters that use USB or Thunderbolt data. But that's not in scope of your question.