So basically you download Magisk on your phone (which hides the fact that the phone is rooted) then go into the Play Store and download the app ?
No, you root your phone with the magisk zip file via custom recovery......as you would with superSU......you will need to unroot the phone first before rooting again with magisk.....you also need the magisk uninstall zip as well as the magisk manager apk.....
you will need have stock boot image too, so if you have a custom rom on the go it may not work....
as far as downloading the app from google play....if it doesn't let you download now, installing magisk wont change that....you can get the apk file for the app off a website....
once installed you go into magisk manager and you can then hide root from each app on your phone separately, which is a cool function....