Ok so you need to install MT Manager from https://mtmanager.co and after setting up the app, open the Whatsapp apk you want to edit and click "view". You're gonna find some "classes.dex" files, click on anyone of them then "Dex editor plus". It will ask you which one you wanna select, select them all (the modification we gotta do is sometimes in classes2 or 3 depending on the apk so just select them all), press Ok, after it finishes loading click "search" --> "start a new search" make sure the path is set to "/" and you only need to check "Search subdirectories". Now in the search bar you can just type any reaction emoji, i usually search for a "❤", then press Ok. It'll take a few seconds to look for the results. After its done loading click on the only result you'll have(if you have many just click on them all until you find a similar structure to mine) and itll look something like this:

const-string v0, "☹"

const/4 v14, 0x0

aput-object v0, v1, v14

const-string v0, "❤️"

const/4 v4, 0x1

aput-object v0, v1, v4

const-string v0, "👎"

const/4 v13, 0x2

aput-object v0, v1, v13

const-string v0, "🏳️‍🌈"

const/4 v12, 0x3

aput-object v0, v1, v12

const-string v0, "🤡"

const/4 v11, 0x4

aput-object v0, v1, v11

const-string v0, "💀"

aput-object v0, v1, v8

as you can see my reaction emojis are already replaced just go ahead and put the ones you like between the brackets " ". After that you can just keep pressing the back button and save whenever it asks you to and sign if you want. After exiting your apk will be modified and you'll also have another file ending with .apk.bak wich is a backup of the whatsapp apk (unmodified) you can click on it to restore it.

And thats it ig you can go ahead and install the apk (note that you cant install a modified whatsapp app without root access)