[Sony Xperia 1 V] UniquePhillip:
This guide will instruct you on how to root your phone and optionally flash US modem firmware (since US version currently can't be bootloader unlocked). These are notes I took for myself after doing this on my DQ54 (EU) but it should be the same on the 72 (Asia) or 44 (Japan) as well.

I have never owned a sony phone before and had to put together these steps from multiple guides. I wanted to share my notes so other people could just follow one guide.

This will be good for you if you have rooted phones before but never a sony phone. It is similar to the 1 IV with a few differences.

Obtaining an international (unlockable) model

If you want to obtain a DQ54 (EU), I suggest you buy from sony.de and have it shipped to a reshipping service. I was impressed, it took less than a week to arrive - and because of currency conversion rates and market adjustments, you will save around $150 even after reshipping fees. Apparently wondamobile is a good source for the DQ72 (Asia), not sure how good the pricing is.

Repairing a soft bricked phone

If this is your first sony phone (like me), something good to know is that if anything breaks, you can often boot into flash mode (plug in USB, then boot while holding vol down) and run xperia companion -> repair, it will save you from many soft brick/bootloop problems.

Drivers

Download NewFlasher and run newflasher.exe in cmd. When it offers to download a driver, select y, then run the file it downloads.

You will also need drivers from https://developer.sony.com/file/download/xperia-1-v-driver/

When you plug in your phone and boot it into any mode other than android (like flash mode), it will show up as an unrecognized mobile device in device manager. Double click it and click the Driver tab -> Update driver -> Browse my computer -> Let me pick from a list -> Any device -> Have Disk... -> Pick the folder you downloaded and extracted.

This second step (not newflasher) needs to be repeated for every state (flashing mode, fastbootd, etc). You use the same driver every time.

Flash US modem (optional) - You don't need root for this

Useful resource: https://xdaforums.com/t/1-v-5g-enable-dq72-tested-us-t-mobile-ongoing.4610273/

Download US firmware with XperiFirm, and copy out the three files that start with "modem" to a new folder with nothing else in it. Download NewFlasher and extract the exe into that folder (the folder should only have 4 files). Boot phone into flash mode (plug in USB, then boot while holding vol down). Open cmd and run newflasher.exe (no flags needed).

Dial *#*#4636#*#* and pick Phone Information -> 3 dots in the top right -> Select Radio Band. Pick United States. Back out and set Set Preferred Network Type to NR/LTE/GSM/WCDMA.

You should see NR Available true and NR State NOT_RESTRICTED

Unlock bootloader - This step is not possible on DQ62 (US)

Before you do this you should dump your trim in case you want to unroot. Download NewFlasher and extract the exe into an empty folder. The folder should be empty except for newflasher.exe or you risk accidentally flashing your phone. Boot into flash mode (plug in USB, then boot while holding vol down). Run newflasher.exe in cmd and say y when it asks if you want to dump the trim.

Once you've dumped the trim, enable developer options like normal and turn on adb debugging and oem unlocking. USB also defaults to charge only so tap the notification and switch to file transfer.

Go to the sony bootloader unlock page and select xperia 1 IV (yes even though this is the 1 V). Put in your IMEI and it will give you a code (you can dial *#*#7378423#*#* to see your IMEI). Boot into fastboot (adb reboot fastboot or plug in USB, then boot while holding vol up), and run fastboot oem unlock 0x(Code) (Btw, if you have US model, this is where you will run into problems. The website will not give you a code)

Your phone will factory reset and reboot multiple times. You will see a warning about how the bootloader is unlocked/corrupt multiple times before android runs, do not worry. It took my phone about 5 minutes.

Root with Magisk

Useful resource: https://xdaforums.com/t/does-sony-blocked-rooting-for-all-variants-of-1-v.4591445/

And also: https://droidwin.com/extract-sony-boot-sin-get-stock-boot-img-file/

Install magisk manager on your phone, download EU firmware with XperiFirm, and download sony_dump.exe into the folder you downloaded the EU firmware into. Find a file named init_boot_[something].sin, and copy the full filename. Open cmd and run sony_dump.exe output init_boot_[something].sin

A new subfolder named output will be created with a .crt file and a .000 file. Change the file extension of the .000 file from .000 to .img. Copy it to your phone and patch it with magisk manager app. Transfer the newly created patched image back to your PC. Magisk will put the new, patched image in /sdcard/Download.

Boot into fastbootd (not fastboot) with adb reboot fastbootd or boot while holding vol up, then run fastboot reboot fastbootd. Then run fastboot flash init_boot magisk_patched-xxxxx.img

Optionally hide root by following these steps (same as other phones)