[Google Pixel XL Questions & Answers] ekjean:
Okay, I've seen similar problems to mine but none of the suggested solutions seem to work out the problem. This seems to be the recommended solution, but it's not getting me anywhere:

https://xdaforums.com/pixel-xl/how-to/info-how-restored-to-stock-soft-t3494478

My bootloader is unlocked. I unlocked it the second I got the phone to save pain down the road. I've now hit the fork. It's an unlocked phone, not Verizon, I'm not sure what the original carrier was, it was second-hand and under no warranty. I'm on Android 10 Marlin, I updated it to the latest about a month ago. I got up to do some laundry last night and came back to a spontaneously dead phone. For detail, I was transferring a movie to my phone via USB to internal storage so I could watch it later. No idea what happened while I was up, but the phone was in the exact same position as when I left. Developer options were on, USB debugging was on.

What I have now is this: It will enter fastboot. When I prompt it to boot to recovery, it shuts down, or it will go to the unlocked bootloader warning screen, allow me to pause/continue booting, at which point it will either go directly to black, or briefly show the Google bootloader and then shut off. When I leave it on a charge, either trickle or full, it will sometimes loop through the unlocked bootloader warning, flicking that screen on and off, but sometimes it's just off. I never see the charging battery screen. I've left it to charge for hours.

Sometimes windows acknowledges a USB device is connected and that it's android, but at no point can I access storage. When working through a command line prompt, acknowledges that the device is there, but trying to flash anything fails immediately. Here is a sample of some of my results.

When trying the flash-all solution:

Sending 'bootloader_b' (32480 KB) OKAY [ 1.779s]
Writing 'bootloader_b' FAILED (remote: 'Verify bootloader image failed on production device.')
fastboot: error: Command failed
Rebooting into bootloader OKAY [ 0.040s]
Finished. Total time: 0.044s

The device goes black. I'll bring it back up to fastboot and the command line spits this out:

< waiting for any device >
Sending 'radio_b' (57156 KB) OKAY [ 3.785s]
Writing 'radio_b' FAILED (remote: 'flash write failure')
fastboot: error: Command failed
Rebooting into bootloader OKAY [ 0.199s]
Finished. Total time: 0.207s

Using fastboot to try and boot factory bootloader, it says it's worked but then the phone again just goes to black and does nothing:

PS C:\Google\marlin-qp1a.191005.007.a3-factory-bef66533\marlin-qp1a.191005.007.a3> fastboot boot bootloader-marlin-8996-012001-1908071822.img
< waiting for any device >
creating boot image...
creating boot image - 33261568 bytes
Sending 'boot.img' (32482 KB) OKAY [ 2.100s]
Booting OKAY [ 0.531s]
Finished. Total time: 2.944s

Flashing that factory bootloader, seems successful, again, back to black screen and nothing until going back to fastboot:

>> PS C:\Google\marlin-qp1a.191005.007.a3-factory-bef66533\marlin-qp1a.191005.007.a3> fastboot boot bootloader-marlin-8996-012001-1908071822.img
>> < waiting for any device >
>> creating boot image...
>> creating boot image - 33261568 bytes
>> Sending 'boot.img' (32482 KB) OKAY [ 2.100s]
>> Booting OKAY [ 0.531s]
>> Finished. Total time: 2.944s

And finally, trying to push an update image over fastboot:

PS C:\Google\marlin-qp1a.191005.007.a3-factory-bef66533\marlin-qp1a.191005.007.a3> fastboot update image-marlin-qp1a.191005.007.a3.zip
--------------------------------------------
Bootloader Version...: N/A
Baseband Version.....: N/A
Serial Number........: HT6AD0202367
--------------------------------------------
extracting android-info.txt (0 MB) to RAM...
Checking 'product' OKAY [ 0.044s]
Checking 'version-bootloader' FAILED

Device version-bootloader is 'N/A'.
Update requires '8996-012001-1908071822'.

fastboot: error: requirements not met!

I am wondering if locking the bootloader would give this thing what it wants and let me boot into recovery, but I have a terrible feeling it would just further lock me out of saving this phone. Does anyone have any insight? Is this a shot motherboard? Sorry, I know my problem has a ton of commonalities with a lot of other people's but it seems for all my searching, reading and Googling that it's idiosyncratic to the point of every solution not working for me. Thank you so much.

Edit: For interest, I just saw the screen throw up an odd block of static, for lack of a better term. In the bottom right corner, it took up a little more than 1/4 of the screen. It looked lik there was cut off text at the top, too little showing to read. It disappeared before I could take a picture with a spare phone. It lasted maybe 5 seconds and then it was gone. It's on USB charge, not sitting on fastboot right now.