Just copy the last slug com.whatsap for an example from play store. I think, it's not grab from play store but from some where else (apkmirror / apkpure, im forgot). I think it's from there. You can try with weird app which not available from play store but it's exist at apkmirror / apkpure (one of them)
Manual apk only for updated version, the first created app / game grab from another source.
For register function, it is use new-account expression not register. It's need a lot of work. But i recommend you to install laravel (which already exist in softcolous installer). From there, you can create this functions (register and login). You may stick in new-account or register? It's up to you.
From here you can start learn how the codes work. Many of paid user can't reach this case. I hope it's clear for you.