Hello, apologies this game is archived using winrar (can be unzipped using winrar or 7z), please follow the steps bellow :
1. Download the purchased file "SB Futanari Birthday ~ Full Version.rar", which is an archive with multiple files
2. Use 7zip or winrar to decompress the downloaded archive (can be freely downloaded https://www.7-zip.org/download.html )
3a. [PC] SB_Futanari Birthday_PC > SB Nishy Birthday.exe
3b. [Phone/Tablet] Upload "sb_BirthDay_En.apk" to the android device, install and play
3c. [Phone/Tablet] if the fps / performance is low (the application is lagging) then upload to android device and install "sb_BirthDay_En_LowQuality.apk"
I hope this helps, if you have any questions feel free to ask
Cheers ;)