[Google Pixel 3 XL Questions & Answers] rootisthebest:
im kinda having the same problem... the phone won't start from power button, starts only in fastboot with volume minus. The phone is detected in fastboot and i can send commands and if I select recovery, black screen and the phone not detected in device manager.

i tried updating and got this

target reported max download size of 268435456 bytes
sending 'bootloader_a' (8489 KB)...
OKAY [ 0.296s]
writing 'bootloader_a'...
(bootloader) Flashing Pack version b1c1-0.1-5034669
(bootloader) Flashing partition table for Lun = 0
(bootloader) Flashing partition table for Lun = 1
(bootloader) Flashing partition table for Lun = 2
(bootloader) Flashing partition table for Lun = 4
(bootloader) Flashing partition table for Lun = 5
(bootloader) Flashing partition msadp_a
(bootloader) Flashing partition xbl_a
(bootloader) Flashing partition xbl_config_a
(bootloader) Flashing partition aop_a
(bootloader) Flashing partition tz_a
(bootloader) Flashing partition hyp_a
(bootloader) Flashing partition abl_a
(bootloader) Flashing partition keymaster_a
(bootloader) Flashing partition cmnlib_a
(bootloader) Flashing partition cmnlib64_a
(bootloader) Flashing partition devcfg_a
(bootloader) Flashing partition qupfw_a
(bootloader) Flashing partition storsec_a
(bootloader) Flashing partition logfs
OKAY [ 0.512s]
finished. total time: 0.811s
rebooting into bootloader...
OKAY [ 0.070s]
finished. total time: 0.071s
< waiting for any device >
target reported max download size of 268435456 bytes
sending 'radio_a' (71148 KB)...
OKAY [ 1.796s]
writing 'radio_a'...
(bootloader) Flashing Pack version SSD:g845-00023-180917-B-5014671
(bootloader) Flashing partition modem_a
OKAY [ 0.293s]
finished. total time: 2.092s
rebooting into bootloader...
OKAY [ 0.070s]
finished. total time: 0.071s
< waiting for any device >
extracting android-info.txt (0 MB) to RAM...
extracting boot.img (64 MB) to disk... took 0.435s
target reported max download size of 268435456 bytes
archive does not contain 'boot.sig'
archive does not contain 'boot_other.img'
extracting dtbo.img (8 MB) to disk... took 0.042s
archive does not contain 'dtbo.sig'
archive does not contain 'dt.img'
archive does not contain 'recovery.img'
extracting system.img (2158 MB) to disk... took 17.741s
archive does not contain 'system.sig'
extracting system_other.img (418 MB) to disk... took 3.477s
archive does not contain 'system.sig'
extracting vbmeta.img (0 MB) to disk... took 0.001s
archive does not contain 'vbmeta.sig'
extracting vendor.img (445 MB) to disk... took 3.620s
archive does not contain 'vendor.sig'
archive does not contain 'vendor_other.img'
wiping userdata...
Erase successful, but not automatically formatting.
File system type raw not supported.
--------------------------------------------
Bootloader Version...: b1c1-0.1-5034669
Baseband Version.....: g845-00023-180917-B-5014671
Serial Number........: XXXXXXXXX
--------------------------------------------
checking product...
OKAY [ 0.060s]
checking partition-exists...
FAILED (remote: variable not found)
finished. total time: 0.362s
Press any key to exit...

thanks for any help

EDIT: My dumb butt wasnt using updated platform tools :(