This detailed step-by-step guide helps you transform your Google Nexus 7 (3G or wifi) with Android 5 to a powerful one with Android 7.1 Nougat.
Don't expect stable Android 8 or 9 on this tab, but 7.1 is already perfect ;-)

### I'll make a few notes using this style: PLEASE READ THEM.

Before going further, make sure you have the correct model.
Go to Settings > About tablet > Model number: Nexus 7

And on tablet's back cover: Nexus is written horizontally (vertically on 2013 model)
### See here if not clear: https://android.stackexchange.com/questions/58273/how-to-distinguish-nexus-7-2012-and-nexus-7-2013
And if you have a SIM card slot on the side, it is a 3G model (codename tilapia), if not, Wifi only alias grouper.

Make sure you have latest official updates.
Connect tab to Wifi network.
Go to Settings > About > Update. Download if update is available.

CHANGELOG

v3. Improve readability + update TWRP + gapps-config.txt (thx to @23deanor)
v2. Minor updates for tutorial + pack
v1. Initial release

1) DOWNLOAD

- djibe Nexus 7 2012 pack: https://bit.ly/djibe-nexus7-2012-packv2
(contains USB drivers r13 + fastboot & adb + TWRP recovery 3.5.2_9-0 + gapps-config.txt by @aaopt + working OpenGapps pico + Magisk root).
### Use Incognito web browser mode if needed to download pack.
Unzip pack on Windows desktop.

For 3G model/tilapia:
ROM LineageOS 14.1: https://forum.xda-developers.com/t/rom-7-1-2-tilapia-unofficial-lineageos-14-1-2021-01-11.3978723/
Download latest build (2021-03-13): https://www.dropbox.com/s/v1p5p5qu8ajsq9w/lineage-14.1-20210313-UNOFFICIAL-tilapia.zip?dl=0
Backup: https://mega.nz/file/wKMGiLoS#oI1Dy9bY3rfYWpzfnzctXbcZKilxWArks0P-f4ncboM
Build 2021-01-11: https://androidfilehost.com/?fid=17248734326145713539

For Wifi model/grouper:
ROM LineageOS 14.1: https://forum.xda-developers.com/t/rom-7-1-2-grouper-unofficial-lineageos-14-1-2021-01-11.4112431/
Download latest build: https://androidfilehost.com/?fid=17248734326145713542

Google Apps are included in my pack (open_gapps-arm-7.1-pico-20190209.zip)
Tablet must be fully charged to 100%.

2) INSTALL WINDOWS DRIVERS AND BACKUP

Go to the folder you unzipped earlier.
navigate to usb_driver_r13-windows > usb_driver.
Right click on android_winusb.inf and select Install. Confirm with OK.
Restart Windows.

Make sure Google/provider account is synchronized.
Backup photos, documents, contacts before going any further (ex: use the backupyourmobile app).
Connect tab to PC and retrieve data.

3) UNLOCK BOOTLOADER OF GOOGLE NEXUS 7 2012

In Android, go to Settings > About tablet.
Tap 7 times on "Build number", a notification confirms Developer menu is unlocked.
Go to Settings > Developer options and check USB debugging ON.

Open a Windows command prompt on the /djibe pack folder you unziped earlier:
select folder with left click, right click while holding SHIFT on keyboard > Open command windows here

Connect tab to PC.

Enter command

adb devices

015d3b668413fc0d        unauthorized

ADB (Android Debug Bridge) has started but tab is waiting for authorization.
On tablet, authorize adb commands from Allow USB debugging dialog: tap on OK.

Then enter command

adb reboot bootloader

Enter command

fastboot oem unlock

Use Volume keys to select Reboot and Power to confirm.
Tablet is unlocked.

Configure Android fast, reactivate Developer options and USB debugging.
Then restart tablet to bootloader using once again

adb reboot bootloader

4) FLASH TWRP 3.5 RECOVERY

Now enter this command

fastboot flash recovery twrp-3.5.2_9-0-tilapia.img

fastboot flash recovery twrp-3.5.2_9-0-grouper.img

Use Volume keys to select Recovery mode, confirm with Power.

Welcome to TWRP recovery.
Check Never show this screen during boot again and Swip to Allow Modifications.

Go to Backup > select System, Data, Boot partitions.
Swipe to Backup and wait during backup.

Back to home screen > Mount.
You can access folders from PC if Data partition is checke and bottom right button show Disable MTP.

From Windows Explorer, navigate to Nexus 7 > Internal Storage.
Copy TWRP folder and paste it somewhere safe on PC.
This is a perfect backup in case of crash.

5) SET FILESYSTEM TO F2FS

F2FS is the fastest filesystem for read and write partitions on flash memories. Compared to EXT4, gain is from 15 to 280%.
No need to format the /System partition because it is Read only and Ext4 is still faster.

Now go Wipe > Advanced Wipe.
Select Dalvik / ART Cache, System, Cache, Data, Internal Storage and Swipe to Wipe.
Tap on Back button (to Advanced Wipe Menu).
Select /cache partition > Repair or Change File System > Change File System > F2FS, Swipe.
Go Back and back to Advanced Wipe Menu.
Select /data partition > Repair or Change File System > Change File System > F2FS, Swipe.
Tap on Home button > Reboot > Recovery > Swipe to Reboot

6) FLASH ANDROID + OPENGAPPS + MAGISK

On restart, check Never show this screen during boot again and Swipe to Allow Modifications.

Back to Mount menu.
Connect tab to computer.
Copy ROM + GAPPS + gapps-config.txt + Magisk
### Do not unzip files. Copy and paste zip files to Internal Storage

### Copy gapps-config.txt or you will get an "insufficient space" crash during flash.

Then go back to Home menu > Install

Select lineage-*.zip, and Add More Zips,
then open_gapps-arm-7.1-pico-20190209.zip and Add More Zips,
then Magisk.zip and Swipe to force install.
Be patient.
### If it fails, install zips 1 by 1.

Android 7.1 is installed !
When done message prompts, tap on Wipe Cache/Dalvik and swype to confirm, wait.
Disconnect tab from PC.
Then Reboot System.
Let the tab start (at least 10 minutes).

7) ROOT WITH MAGISK

Configure Android as you like.
Magisk root starts with Android.
You can manage permissions and extra modules with Magisk Manager app.

Open Magisk app (v23+).
Go to Settings > go to App Rename app to Godmode to hide hide.
Below, switch MagiskHide ON.
Now go to Magisk Modules tab > install liboemcrypto disabler and MagiskHide Props Config.

Connect to your Google account, download from Play Store Netflix and Disney+.

### If you are already connected to Google account.
Clear cache (Settings > Apps > All apps > Google Play Store > Storage > Clear storage) and restart tab.
Relaunch Play Store, Netflix and Disney+ will be available in SD.

10) OTHER APPS

- You can use GPS Status & Toolbox for faster GPS fix.
- Developer options
Settings > About tablet.
Tap 7 times on build number.
Back to Settings > Developer options
Select USB Configuration > set MTP
Set Windows animation scale, Transition animation scale and Animator duration scale to .5x

- In Developer options, you may try Force GPU rendering.
Much faster on my tab, tell me if it does (or not for you)

That's it.

Enjoy ;-)

Thanks: ud4, AndDiSa, aaopt, TWRP team, LineageOS team, OpenGApps team

3 choices:

1. From within the /flash folder, enter "cmd" in the address bar and hit ENTER. It will open the CMD in the /flash context.
2. Copy path of folder from Windows Explorer address bar and enter following command in CMD:
   cd /d (CTRL + V TO PASTE THE PATH)
3. Move to the parent directory (/djibe). Select /flash folder, then hold SHIFT and right click Open command here.
   Command will open on the /flash context.

If it doesnt work, go over Windows Start Menu icon > right click > Device management and make sure the Tab is well detected.