The apk file should not more than 25M, if with big file size, customer will download it, it will waste much internet and time.
Push notification by firebase
When open the app, show the splash screen first, after that show the ads photo that we can control by replace the pictures in our sever. The ads photo show just in 4 seconds and also should have the skip button to skip it.
After the app shows the ads picture, it will show the login page, and in the login page, it just shows the username and password, so the server name just fixed in the app by using a domain name instead.
If the user was registered in one device, then can not register in another device.
Even if the androip app not open, it still can get the incoming calls. （Firebase）
App update notification (Firebase)
When upload or publish to the store (both google playstore or APPLE STORE), use teamviewer or anydesk do it remotely in our computer.