[Samsung Galaxy A12] juliano-oh!:
Hello, today I'll be guiding you to how to install LineageOS or other GSI ROMs on your A127F U7 device.

First, some precaution:

/*
* knox_warranty_void: 0x01
*
* Your warranty is now void.
* I am not responsible for bricked devices, dead SD cards,
* thermonuclear war, or your getting fired because the alarm app failed. Please
* do some research if you have any concerns about features included in this ROM
* before flashing it! YOU are choosing to make these modifications, and if
* you point the finger at me for messing up your device, I will laugh at you.
*/

Requirements:

* Unlocked bootloader
* Your device's firmware
* Android's platform tools.
* Magisk
* Odin
* U5, U6, U7 TWRP
* GSIs Image for the ROM you want to download
* physwizz's kernel

1) The first step you want to do is, unlock your bootloader. You can simply do that by enabling OEM Unlocking in Developer settings, (Can be unlocked by going to Settings > About Phone > Software Information > tapping "Build Number" 5 times then booting to download mode. (Connect to PC, shutdown your phone, then hold volume up and down buttons, then when you're on the download screen, hold volume up., then press volume up again) WARNING: THIS WILL ERASE YOUR DATA AND PERFORM A FACTORY RESET, SO BACKUP YOUR DATA WHEN NECESSARY.

2) Then, download the firmware you have for your firmware. How? Try visiting sites like samfw and find your firmware by searching A-127F and finding your country's baseband. Then download through your preferred method of downloading. Mine I login to Google, and downloaded through Google Drive, which is faster. (Though it will personally copy the firmware's file to you Google Drive and occupy space on it, so beware of that and delete the file afterwards, the file is in DRIVE > samfw_temporary_folder_dont_delete I assume it's safe to delete after download, so there's that.)

3) Extract the file, and adb push the AP_XXXXX file to your device. How? Download the Android platform tools to the above hyperlink, then extract it, go to the file and then type: e.g adb push [DIRECTORY WHERE YOU EXTRACTED YOUR FIRMWARE FILE]\AP_XXXXXX, [FILL IN THE FILE NAME] /sdcard/

4) Then boot up your device, set it up (minimally), download the magisk from below, (25.1-delta file) then install it. Then from the app, select Install Magisk > Patch from File then select the AP_XXXX file you've extracted from your firmware. Wait, for some minutes, and then see what is the file name.

5) Then pull the file and flash with Odin. (Download below) Use adb pull /sdcard/Download/magisk_patched-XXXX [See the magisk log for the file name] C:\Users\[your user]\Downloads\. Open Odin, select okay, when prompted, select AP, select the magisk_patched-XXXX file you've extracted from your phone, untick auto-reboot, then hit flash.

6) Go to stock recovery, again by pressing Volume Down +Power, then while connected to the PC, press Volume Up + Power then navigate via volume buttons and then press with power button, Wipe data/factory Reset

7) Turn off the phone (while connected to the PC) by pressing Volume Down + Power, then when turned off immediately press Volume Up + Volume Down. Go back to Odin, download the TWRP image below, and put it in AP slot and untick auto reboot. Then flash

8) Boot to recovery, by again Volume Down + Power, when turned off, Volume Up + Power, whilst connected to PC. ( See 6.) Now you've booted to TWRP Recovery. Go to Advance > Terminal then type multidisabler twice. (Very important)

9) Then download the GSI ROM you want to flash, extract it, then adb push it to your /sdcard/, just as you did with 3. (Here is a list of GSI's you can download) (Optional) Either use MTP or adb push to get into your device physwizz's kernel, and GApps (the guide will be in the second post, yet to find a way how to install it)

10) In the recovery, select Install > Install Image > select the GSI ROM's .img file you have, and select System Partition. (Optional) Then select Install Zip Then install physwizz's kernel.

11) Then, reboot, WAILA! There you have it, your own GSI ROM on your A127F U7 device. Enjoy

Credits: @physwizz, for his recovery and kernel ports. Without his hardwork, it wouldn't even be possible to install GSI for U7