[kodithegreat]

Does not respond to any physical commands (bootloader, recovery, etc) 
Screen is constantly black.
PC recognises mobile as Qualcomm connection and will disconnect and reconnect if I hold the power button/volume keys down but mobile screen remains black.
ADB and Fastboot cannot find device
Phone was 80% charged and working the night before but was like this in the morning.
2 Weeks prior installed Android 11 R1 with magisk canary root and ElementalX kernel. 
Had 2 modules that ran fine, one for Google pay and the other for call recording.
Attempted Miracle Box Qualcomm methods without the firehose as I cannot find any online footprint for it. 
The only readable info was this CPU output from MiracleBox (Thunder Edition 2.82) whilst in Read info EDL/Diag Mode - Universal OnePlus Method
CPU:0x0008b0e1

If anyone can help me out that would be great, I can't RMA as I am not covered by warranty because the screen has a minor crack. Google's third party repairer wants $500 just to replace the front screen and back housing.

[X3non]

...

unless you can find the right firehose programmer file for this model, there's not much you can do and it's very unlikely that you'll find it
if you have access to another pixel 3 xl, you could try the method @ https://www.hovatek.com/forum/thread-19721.html ; see if the device accepts booting up from ext_sd

[kodithegreat]

Unfortunately the Pixel 3 XL does not have an SD Card slot >

[X3non]

Unfortunately the Pixel 3 XL does not have an SD Card slot >

guess only the OEM can really help you now