[LG V30 ROMs, Kernels, Recoveries, & Other Developm] boredz:
Thanks you for your time, but I don't know, maybe I'm not clear enough or I don't really understand what you're all trying to say (more than a possibility lol), this is exactly what I'm doing:

Pastebin version with numbered rows, use it, I'll refer to the exact rows below.

Plain text:

PS C:\Users\me\Downloads> adb reboot bootloader

PS C:\Users\me\Downloads> fastboot boot .\twrp-3.1.1-0-h930.img
downloading 'boot.img'...
OKAY [ 0.541s]
booting...
OKAY [ 0.023s]
finished. total time: 0.565s

--- recovery ----
PS C:\Users\me\Downloads> adb shell

~ # whoami
whoami
root

~ # cp /system/fonts/NotoColorEmoji.ttf /system/fonts/NotoColorEmoji.ttf.old
cp /system/fonts/NotoColorEmoji.ttf /system/fonts/NotoColorEmoji.ttf.old

~ # cp /external_sd/NotoColorEmoji.ttf /system/fonts/
cp /external_sd/NotoColorEmoji.ttf /system/fonts/

~ # ls /system/fonts/NotoColor* -l
ls /system/fonts/NotoColor* -l
-rwxrwxrwx 1 root root 8314572 Mar 3 00:21 /system/fonts/NotoColorEmoji.ttf | different file sizes
-rwxrwxrwx 1 root root 5915824 Mar 3 00:19 /system/fonts/NotoColorEmoji.ttf.old | different files.

~ # chmod 644 /system/fonts/NotoColorEmoji.ttf
chmod 644 /system/fonts/NotoColorEmoji.ttf

~ # chmod 644 /system/fonts/NotoColorEmoji.ttf.old
chmod 644 /system/fonts/NotoColorEmoji.ttf.old

~ # ls /system/fonts/NotoColor* -l
ls /system/fonts/NotoColor* -l
-rw-r--r-- 1 root root 8314572 Mar 3 00:21 /system/fonts/NotoColorEmoji.ttf
-rw-r--r-- 1 root root 5915824 Mar 3 00:19 /system/fonts/NotoColorEmoji.ttf.old

~ # exit
exit

PS C:\Users\me\Downloads> adb reboot

--- ROM ---
PS C:\Users\me\Downloads> adb shell

joan:/ $ whoami
whoami
shell

joan:/ $ ls -l /system/fonts/NotoColor*
ls -l /system/fonts/NotoColor*
-rw-r--r-- 1 root root 5915824 2008-12-31 19:30 /system/fonts/NotoColorEmoji.ttf

joan:/ $ exit
exit

PS C:\Users\me\Downloads> adb reboot bootloader

PS C:\Users\me\Downloads> fastboot boot .\twrp-3.1.1-0-h930.img
downloading 'boot.img'...
OKAY [ 0.523s]
booting...
OKAY [ 0.021s]
finished. total time: 0.545s

--- recovery ---
PS C:\Users\me\Downloads> adb shell

~ # ls /system/fonts/NotoColor* -l
ls /system/fonts/NotoColor* -l
-rw-r--r-- 1 root root 8314572 Mar 3 00:21 /system/fonts/NotoColorEmoji.ttf
-rw-r--r-- 1 root root 5915824 Mar 3 00:19 /system/fonts/NotoColorEmoji.ttf.old

~ # exit
exit

As line #16 shows, I'm actually root, I backup the file and copy the new one in /system/fonts (#18, #21), as lines #26 and #27 point out I have two very different files (different sizes), I change the files permissions (#29, #32) and the output of #35 is actually what I want.

I reboot the device into the ROM, at line #51 you can see I'm not root anymore, and the command at #53 just prints ONE file, the ORIGINAL one (note that the file size is exactly the NotoColorEmoji.ttf.old's one), but I should be able to see both (chmod 644, everyone can read/see those files).

I boot again in recovery, give command at #73 and it says that there are TWO files, the ones I want to have.

Why? It seems like TWRP is working on a virtual file system of its own.

Yes, I have mounted /system as read and write, otherwise it gives an error when you try to access it.

Also, please note that I still see the stock Android emojis, not the ones contained in the new file (the 8MB one), and even if the file is not supported/broken/whatever I should see nothing, not the stock emojis.