[Google Pixel 4a 5G Questions & Answers] lmt1979:
(Problems I had when attempting a "dry" flash and patch:)

Just got the phone today (November 18) then attempted to flash the boot.img after patching it with magisk manager (after magisk patches the boot.img it renames it to "magisk_patched.img"). Got the usb debugging authorized after updating my adb drivers. If you want to make sure your phone is authorized you may need to run commands: adb kill-server and adb devices to check and stop/restart adb.

Anyone able to get it flashed? Getting error:

FAILED (remote: Failed to write to partition Not Found)

after running

fastboot flash boot magisk_patched.img

from fastboot, bootloader is unlocked. Running version

bramble-rd1a.200810.022.a4 (October)

I'd recommend patching up to the November update: Settings, System, System Update to the newest November update (11.0.0 (RD1A.201105.003.C1, Nov 2020).

EDIT: I'd recommend installing 15 second ADB https://xdaforums.com/showthread.php?t=2588979, then overwriting it with the newest adb tools from google. 15 second ADB will add the ADB folder to your system's PATH (ENVIRONMENT VARIABLES) allowing you to run adb commands from any folder (not just the folder adb resides in).

Ok, updating to usb_driver_r13-windows.zip and platform-tools_r30.0.5-windows.zip and updating the build to the Nov RD1A.201105.003.C1 will report back lol

EDIT 2: So yeah, update your drivers and platform tools, successfully patched it:

fastboot flash boot magisk_patched.img

OUTPUT:

Sending 'boot_b' (98304 KB) OKAY [ 2.310s]
Writing 'boot_b' OKAY [ 0.480s]
Finished. Total time: 3.122s

How to flash and patch:

https://www.xda-developers.com/google-pixel-4a-how-to-unlock-bootloader-root-pass-safetynet/

Prerequisites:

Download the applicable factory image https://developers.google.com/android/images The Pixel 4A 5G is codenamed "Bramble" (Pull down your shade or look in Settings, "About phone" for Build number) and extract the boot.img from factory image, then transfer it to phone and patch it with magisk manager:

Open Magisk manager -> select ‘Install’ -> ‘Select and Patch File’ -> select your boot.img file

The patched boot image should be found inside your Download folder

After magisk patches it, mount your phone's storage, copy to your computer and flash it from fastboot. Hold down the Power and Volume Down buttons to enter fastboot.

Run command

fastboot flash boot magisk_patched.img

Platform Tools (includes ADB)

https://dl.google.com/android/repository/platform-tools-latest-windows.zip

Usb Drivers

https://dl.google.com/android/repository/usb_driver_r13-windows.zip?hl=ru

Magisk

Direct Link https://github.com/topjohnwu/Magisk/releases/download/v21.1/Magisk-v21.1.zip

Direct Link https://github.com/topjohnwu/Magisk/releases/download/manager-v8.0.3/MagiskManager-v8.0.3.apk

Magisk Releases page (in case of an update to magisk or magisk manager) https://github.com/topjohnwu/Magisk/releases/

VIPER4Android (great Digital Signal Processor)

I have been struggling with getting Viper4Android installed.

Stop-gap here:

I flashed this one, but it's distorting my external speakers and is buggy, but it does process...so BE CAREFUL. This will get you there until a more stable version is out. And the video was just taken down. All you have to do is flash it with Magisk here's a link:

https://drive.google.com/file/d/17-NTL0miBYzk4o4v26MstAL9lOPXF3GN/view?usp=sharing

AdAway (system-wide ad blocking)

tried version 5.10 with no luck, let me know if it works for you.

AdAway 4.34 working for me on Android 11.0.0 (RD1A.201105.003.C1, Nov 2020)

Here: https://f-droid.org/repo/org.adaway_40304.apk

AdAway 4.36 (haven't tested)

https://f-droid.org/repo/org.adaway_40306.apk

Make sure to enable "Systemless Hosts" in Magisk Manager before attempting to enable ad-blocking. It's under Settings (the cog/gear in the top right) in the Magisk section.

Good luck all!