After taking a closer look at USB-C or Thunderbolt 3 and how they achieve video output, I don't think an adapter exists to connect an HDMI video source to connect to a USB-C display. Even the official HDMI website shows the conversion as being one-way: USB-C to HDMI.
A major caveat is that sending HDMI over USB-C requires the video source to send additional signals that are not part of the HDMI specification; They're part of the USB-C specification.
For conversion in this direction to work, an adapter would have to be a USB-C Host controller and signal to the monitor that it wants to use the HDMI alternate mode. Creating a video adapter that acts as a USB-C host device is not trivial, and almost pointless since most devices or screens have multiple video connectors to use instead.
I believe that it may be possible to make such an adapter, but the demand for it is too low for any to exist yet.
"This is 2019." Yes, but new video connectors don't always make things simpler.
To answer your question seriously, the thunderbolt 3 (USB-C) connection uses a DisplayPort signal for video. Unfortunately HDMI to DisplayPort (in that direction) is not compatible with just a simple wire-to-wire converter. You actually need something known as an "Active HDMI to Displayport Adapter", and obviously the displayport side needs to be the USB-C connector.
I am also fairly certain that the USB-C monitors get their power from the USB-C connection. If you have a monitor like that, the adapter would have to supply power too!
I have never seen an adapter that meets all of those requirements so I cannot recommend one for you.
It might be possible to combine two adapters:
- HDMI -> DisplayPort
This one must be active, so requires its own power usually from USB.
- DisplayPort -> USB-C
This one may be VERY hard to find in this direction but theoretically possible. (It probably doesn't exist.)
It turns out that I entirely missed that USB-C (and thunderbolt 3) can output HDMI signals too (as an Alternate Mode on top of USB-C).
That implies that USB-C displays (but not necessarily ones that require Thunderbolt 3) can accept HDMI signals through that connector. This raises some hope that there is a passive HDMI to USB-C cable/adapter out there. (This isn't possible without negotiating the video mode over USB first.)