[Walmart Onn Tablets General] netninja2001:
First, to clarify, the last time I rooted anything was a Galaxy S3 back in the day. I'm by no means an expert, I just figured I'd help out folks like me that haven't touched all these new tools, well, ever.

Just did this myself earlier today attempted to get Magisk root, but alas, I got stuck in a boot loop. Luckily I had muddled through getting the image first, otherwise I'd still be stuck in said boot loop. I figured I'd post this for anyone else trying to root these things just to make sure you have a backup you can trust (I generally don't trust rando images that folks post online). All of the instructions below are assuming you're on Windows 10 and using PowerShell just because that's the default these days. Without further adieu.

Download ADB/fastboot (on your Windows machine)

I downloaded adb/fastboot from google directly: https://developer.android.com/studio/releases/platform-tools

Just extract and browse to the platform-tools directory in Windows Explorer until you can see adb.exe and a bunch of other tools

Download the latest SP Flash Tool

I just grabbed it from here: https://spflashtools.com/

Please let me know if there's an "official" place to find SP Flash Tool, cause everything surrounding all the download sites seems a bit sus...

Enable USB Debugging (on the tablet)

Go to settings -> About tablet

Tap the Build number 10 times (until debugging mode unlocks)

Hit back and go to System

Click Advanced and then Developer Options

Enable USB Debugger

I also enabled OEM unlocking because the whole point of this is for me to run either AOSP or Lineage someday. I honestly don't know if unlocking the bootloader is needed for dumping your own images, but I highly suspect it's not.

Plug the tablet into your computer

You'll likely see a prompt on your tablet about allowing your computer to debug your tablet. I just checked the box and hit accept so I wouldn't see it again.

Get your scatter file (on your Windows machine)

I tried several things to get the scatter file (I guess this is like a partition table based on the contents I saw) but in the end, by far the easiest way was to just download the scatter file from the file system.

In the Windows Explorer window from before (platform-tools), hold down shift while left clicking and click on Open PowerShell window here

Now type the following to get a shell on your:

.\adb.exe shell

This will get you into the shell environment. Now type the following to verify your scatter file is there:

ls -al /system/data/misc/

In here you should see something like:

-rw-r--r-- 1 root root 13893 2008-12-31 19:00 MT8168_Android_scatter.txt

Now that we have the name, just type exit to get out of the shell

Download the scatter file

.\adb.exe pull /system/data/misc/MT8168_Android_scatter.txt

Open up the scatter file in your favorite text editor (for me, Notepad++)

Now you can see the partition layout, offsets, etc, etc

Dump your image(s) (on your Windows machine)

Now open SP Flash Tool (flash_tool.exe)

On the Download tab, make sure the Download-Agent is MTK_AllInOne_DA.bin

Now click Choose for the Scatter-loading file and browse to the scatter file you just downloaded

This should be in your platform-tools folder unless you moved it

Once the scatter file is loaded, the partition table should fill up with a bunch of partitions

Click on the Readback tab

Click Add

Double-click on the new entry

Navigate to where you want to save your image, and give it a name (in this case I'm starting with boot.img)

Remember how you opened the scatter file in a text editor? Search in the scatter file for boot.img

Make sure the region matches (should be EMMC_USER) between the scatter file and SP Flash Tool

Copy/paste the value for start_addr in the scatter file to Start Address in SP Flash Tool

Copy/paste the value for partition_size in the scatter file to Length in SP Flash Tool

Now do the same thing for recovery.img and any other images you'd like

If you want a full ROM backup, name the file something like ROM_0, then use Start Address of 0x0 and length as the start_addr for the second-to-last entry in the scatter file (in my case, it was 0xc1a80000, just make sure it doesn't start with f's)

Note: I don't know for sure if this is accurate or not, I'm still playing with it, but so far it appears to be. Probably?

Poking around in WwR MTK 2.51, it looks like for this particular device (100011885) I wanted a total dump of 0x73A000000. It looks like this number is derived from the first 8Mb of the EMMC_USER dump, so I'm not sure of an easier way than throwing WwR MTK at it for the moment.

Once you've got all the entries for what you'd like to dump, make sure to disconnect the tablet from your computer and power it off

Now click on Download in SP Flash tool

Once things grey out, then plug the tablet in. After a few seconds, you'll see the images start dumping.

Congrats, you have a boot.img (and whatever other images you wanted). Like I said, from here I tried using Magisk to patch the boot file, but when I flashed it in fastboot, after enabling OEM unlocking in Developer Options, it just kept popping up the initial Onn graphic along with the Orange State warning without getting to the "fancy" Onn graphic and the rest of the boot process. I was able to flash the original boot.img back and it once again booted properly again.

Also, for anyone interested, I've posted my dumped files for the 100011885 in my google drive: https://drive.google.com/drive/folders/17LtLtjKg4JJU9EJdIXPsyNjen0H-ilMX?usp=sharing

Maybe someone will have pity on me and figure out why Magisk isn't working? :D

Whenever I get a moment, I'll dump my 100003562 as well.