Help me I'm confusion

My nephew wants to play pokemon mmo on his android, I've downloaded the apk but now I'm struggling with the roms side of it. I've found a few reddit posts woth roms to download but can't work out which ones I need as all the versions (eg. Fire red, leaf green) come together and I'm assuming I only need one? I've done skyrim modding most of my life and this isn't the first apk I've downloaded to a phone but past that I'm abit of a noob when It comes to this sort of stuff. Could someone point me in the right direction or a step by step guide to getting this sorted for him. I'm sure most of you are sick of seeing these posts on the reddit and answering the same questions but I swear I've looked around and just can't work it out. Good RNG luck to all, thank you in advanced.