TWRP Installation on Asus Nexus 7 3g (2012), tilapia.

The following applies to the Asus Nexus 7 3g 2012, "tilapia" model, but I suppose that the procedure is the same for the "grouper"--WiFi only--model.
My Nexus 7 had only official Asus updates (Asus Lollipop Android), non-rooted.

The "recovery" is a boot-time utility useful for OS installation, and performing OS backups.

I used my Linux PC but I think that this procedure applies also to Windows and iOS, only with minor changes.

If you broke your Nexus 7 following this guide it's your fault, not mine.. I'm not responsible in any way. This procedure worked for me but software is too much complex for 100% confidence.

Installing TWRP recovery

You need Android-SDK installed on your PC, and for Windows you need also Andorid USB drivers.

In case your Nexus fastboot is locked (normal situation) the following operation is going to delete the user data on your Nexus 7.. backup, backup, backup.

Download TWRP (for "tilapia") image on your PC from https://twrp.me/asus/asusnexus720123g.html (TWRP for  WiFi only "grouper" is here https://twrp.me/asus/asusnexus72012wifi.html), let's download the twrp-3.x.x.x-tilapia.img file on your PC-- there are also apk packages, let's ignore them.


With your Nexus 7 Tablet on, enable USB Debugging: Settings > Developer Options > USB Debugging.
Sometimes, you have to enable Developer Options first:

1. Go to "Settings".
2. tap on "About phone" or "About tablet" entry,
3. Scroll down to the bottom of the "About pho☺ne" and locate the "Build Number" field
4. tap 7 times on "Build Number"

Connect the tablet to your PC with USB cable, reboot to the bootloader using this command from your PC (adb and fastboot are commands installed with Android-SDK):

adb reboot bootloader

Please note that you need to authorize debugging from your PC when performing the previous command.
(Optional: you can also perform this operation manually, the procedure is described later)

Then, unlock the fastboot (this operation invalidates the Nexus guarantee. :-) ):

fastboot oem unlock

A confirmation screen appears on the Nexus  7 screen, you have to select Yes and click "Enter" that is the Power button :-)

Then install the TWRP recovery:

fastboot flash recovery twrp-3.x.x.x-tilapia.img

Now, you have to reboot into recovery immediately in order to persist the change, you  can reboot the device from the bootloader menu or with the command fastboot reboot.
During the reboot, you have to be very quick in order to access the bootloader again and perform immediately (manual way to enter the bootloader):

• press and hold the Power key, and the Volume Down key. 
• You are again in the bootloader, scroll down twice to select Recovery
• Select with the power key.
• you are in TWRP:

Begin "Please Note"
there are lot of ways to perform the same operations. For example you can reboot in the bootloader with the command: fastboot reboot bootloader.
Perhaps you can also try:  fastboot reboot recovery
But I simply described the operations I performed.
End "Please Note"

Now you are running TWRP, I suggest to use TWRP to perform these operations:

• backup the original Asus Android OS (I put buckups into the Nexus /data/media/0/Download/ folder)
• use the adp pull (google is your friend) command to transfer the backup from /data/media/0/Download/ to your (USB connected) PC
• Perform a factory reset (TWRP Wipe)
• ..and finally install (Install button) a new preferred Andorid release for tilapia devices

In order to install a new Android version you have to copy the Android OS image, optional: gapps zip package, optional fix packages and rooting packages in the Nexus SD or internal storage (I usually use /data/media/0/Download/ folder). When running TWRP, the Nexus (connected with USB) is accessible by PC with the adb command or,hopefully ,like USB storage (or MTP).

In detail, I suggest to install the Android AOSP 7.x.x AndDiSa release (xda thread with instructions):

https://forum.xda-developers.com/nexus-7/development/rom-android-7-aosp-grouper-t3467514

Please note: for the proposed SetupWizardFixCrDroidNexus7.zip fix was mandatory, I think you need to use it too.

Have fun,

Quoting my "success post":

I have successfully installed the aosp_tilapia-7.1.2-ota-eng-20181112.ds.zip on my Asus Nexus 3g 2012 (tilapia), I also installed open_gapps-arm-7.1-pico and Magisk v17.3.
I installed your Android release, by using TWRP-3.2.3-0-tilapia, over the Asus original fully updated Lollipop. In fact the most tricky part was installing TWRP

During installation the initial OS configuration failed, multiple times (I tried with reboots), just during the user account selection. Using the suggested SetupWizardFixCrDroidNexus7.zip fix, fixed the problem, and I was able to complete the device configuration.

Finally:
My Nexus is back to life, is very responsive, browsing and playing movies are very fast...
I have not tried all the functions (not tried camera yet).

With official Lollipop my Nexus was simply unusable, u-n-u-s-a-b-l-e, I used to think that my beloved Nexus was simply too old for the new applications. I think now that Asus(or Google, it is a Nexus device) released defective OS versions for manufacturer-planned-obsolescence