Why not from Google Play
The first thing that surprises a beginner: the 888starz app is not in Google Play in Egypt, because Google’s store does not allow real-money betting apps in many countries. So on Android it downloads as an APK from the official link, and the phone asks for your approval once to install from unknown sources. That is normal - what matters is that the file comes from the official link, not from a group or a file site.
On iPhone
On iPhone it is easier: the app is in the Egyptian App Store as “888Starz: Sports Betting”, for iOS 15 or later. Tap Get and confirm with Face ID or your password like any free app. No files and no extra steps.
The first time you open the app
After installing, the first three things: register or log in with your account, set a deposit limit in the account settings before any deposit, and decide on notifications: result notifications are useful, but you can switch offer notifications off if you notice them pulling you back into the app unplanned. Then carry on with your first-week plan from the day you have reached.