dreamliner
These are examples, not definitive choices, to describe what's going on:
Basically, the Vertex tells the source device (like the ATV4K) that the display is Dolby Vision capable AND requires the source to due the dynamic decoding of the DV metadata. The Vertex lets you tell the source device "Hey, send me content optimized for a 1000 nit display" and or other characteristics.
Then the source sends an HDR10 signal that has been optimized by the player for the characteristics you have defined.
Most folks have the Vertex tell the source device to send an HDR10 signal optimized for a display that reach 1000 nit peaks. (I can't tell you how to set your Vertex for that since I have an older model. But I think at this point it is pretty much two or three check boxes in the Vertex setup.)
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Then, on the display side, either you have a built in HDR10 curve already (most projectors these days do), or you have a custom one, that is designed to accept and display content mastered for a 1000 nit capable display. Some displays have a slider that let you dial in the right custom curve to mimic a 1000 nit display. (Your JVC and mine do not so we have to build a custom curve, just like in the old days with the Arve tool, or download someone else's that is "close enough".) And then, in theory (and in practice for most people) you are getting the best possible viewing experience from an HDR signal. The display is optimized for a 1000 nit signal. And the player is processing on the fly to optimize the output for a 1000 nit capable display.
I don't know that the process for creating a custom HDR10 curve for a JVC400 has changed from when you explored it before, but I'd guess maybe checking with other users in the owners thread would be a good way to find out. The good news is that you should only need one curve (that matches the settings you used in the Vertex.)
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As I say, there are lots of nuances one can delve into but that is the general system.