The way that system is wired doesn't leave you any ideal option.  As NVIDIA Control Panel is indicating, the HDMI output on that system is wired to the NVIDIA GPU, and the USB-C/TB3 output is wired to the Intel GPU.  When your display is connected to the latter, the NVIDIA GPU can still be active through NVIDIA Optimus, which involves the NVIDIA GPU acting as a render-only device that sends completed video frames to the Intel GPU for passthrough to the display, but that passthrough design does introduce some limitations.  However, it's not clear exactly what you're seeing in this "night and day difference" compared to systems that are "truly outputting 144 Hz".  If Windows is saying that the Intel GPU is outputting 144 Hz, then it is; it's not just making that up.  However, it's possible that NVIDIA Optimus itself is limited to 60 Hz, which might prevent 144 Hz from being used when the NVIDIA GPU is active.  To be clear, I'm not sure that limitation actually exists; it's just a guess.  I do however know that you can't use G-Sync through Optimus though, so if the other system you're testing has G-Sync enabled (and the display supports it), then that might account for the difference you're seeing.

As for HDMI, my guess as to why you're limited to 60 Hz there is either because the HDMI output doesn't support 144 Hz or the display's HDMI input won't accept 144 Hz.  Some displays can only be driven at their maximum capabilities (resolution, refresh rate, G-Sync, etc.) via their DisplayPort inputs rather than HDMI.

If you're wondering why the USB-C/TB3 output is wired to the Intel GPU at all, it's mostly for battery life reasons.  If it were wired to the NVIDIA GPU, then that GPU would need to be kept running whenever an external display was connected to that output, even if there was nothing graphics-intensive going on.  Wiring the output to the Intel GPU allows the NVIDIA GPU to be completely disabled except when it's needed.  On most laptops with dual GPUs, all outputs are wired to the Intel GPU for this reason.  However, some gaming-oriented systems sometimes wire some or all external outputs directly to the discrete GPU in order to avoid the limitations I described earlier that get imposed when using Optimus passthrough.  For example, running through an Intel GPU means you can't use VR, G-Sync, 5K resolution, or stereoscopic 3D, and possibly some other things.  And then there are very high-end workstation models like the more recent Precision 7000 Series models that have a BIOS option allowing you to choose which outputs are wired to which GPU, but that's only available because those systems use a more complex and expensive motherboard design that involves the outputs being connected to multiplexers that are then wired back to both GPUs, so the user can choose which "path" should be used.

i wanna do tge same thing but im worryed at the fact that i wont beable to do 144hz when gaming because ik that i have a 1060 graphics card tjat can out put it but idk if i can really out put it when gaming i can get a thunderbolt to display por adapter and a hdmi 2.0 cord to test it when i do get the monitor if not imma just replace it completly because there have been serious performance issues w this laptop along w all of the extra dell stuff i dont need.

Just wanted to note that I purchased a TB3 to display port cable from Amazon and am currently using it successfully to output to a 144hz monitor on my G7. I looked through this thread and others and was hesitant to try it but I am glad I did. It will show up as intel graphics still but the rendering is still done on GPU. I have not noticed any performance issues.

I have a G7 from 2018.

I was able to get 144hz working by using a thunderbolt 3 -> displayport cable using the intel graphics control panel application.  I was not able to get Freesync / G-Sync working, however.  A little disappointing, but better than nothing.  With an HDMI cable, it appeared I was able to get 100-120hz.

I don't have much experience with >60hz displays, but fps apps are also showing 100-144fps. 

@Emidica  and @badandy1234  , technically it's a USB-C to DisplayPort cable since those cables don't require the host system to support Thunderbolt 3 and won't take advantage of Thunderbolt 3 even if it's available.

As for G-Sync/FreeSync not working, G-Sync requires the NVIDIA GPU to have direct control of the display output, and in those systems the NVIDIA GPU operates via NVIDIA Optimus, basically as a render-only device that passes video frames to the Intel GPU for output, because the Intel GPU has direct control of the outputs.  The main reason for this design is battery life, since it allows the NVIDIA GPU to be completely shut off when not needed.  If the NVIDIA GPU had direct control of the output, it would need to remain active whenever a display was connected to that output, even if nothing graphics-intensive was going on.  The drawback to the Optimus setup is that certain technologies that the Intel GPU doesn't support passing through and/or that require the NVIDIA GPU to have direct control don't work, such as G-Sync, Adaptive V-Sync, VR, stereoscopic 3D, 5K resolution, and possibly others.  As for FreeSync, to my knowledge Intel GPUs don't support that at the moment, but I'm not 100% sure on that.

@Saulo Amaral  in terms of the issue being discussed in this thread, a DisplayPort to USB-C cable is the same as using a DisplayPort cable plus a USB-C to DisplayPort dongle.  The important factor is how the USB-C port is wired internally, which doesn't change based on the type of cable you connect to that port.  I personally prefer DisplayPort to USB-C cables though, for a few reasons.  First, a dongle is relatively bulky and sits right next to your system, whereas a cable doesn't have that bulk.  Second, a cable+dongle setup is two possible sources of issues, compared to a cable that's just one possible issue.  And third, if the dongle+cable setup was significantly cheaper than a DisplayPort to USB-C cable, then I might consider it, but I've generally found that just the dongle without the DisplayPort cable you would also need costs roughly the same as a DisplayPort to USB-C cable.

Thank you for the quick and thorough reply!

Since 144hz Monitors usually come with a DisplayPort cable and considering that the average price of a DisplayPort to USB-C cabls is three times more expensive than a dongle, I wanted to consider this option. Nevertheless, indeed it is risky to have another variable in this equation which seems to be already tricky. lol

Before finding this discussion I thought it was impossible to have 144hz using a Notebook without DisplayPort (in my case a Dell G7) and now at least I have some hope!

Would it still work if I were to wire a DisplayPort cable to a USB-C port using an adaptor? Or having an actual DisplayPort > USB-C cable is better?