The official guide says:
On Android 4.1 and lower, the Developer options screen is available by default. On Android 4.2 and higher, you must enable this screen as follows:
- Open the Settings app.
- (Only on Android 8.0 or higher) Select System.
- Scroll to the bottom and select About phone.
- Scroll to the bottom and tap Build number 7 times.
- Return to the previous screen to find Developer options near the bottom.
I'm using Android 8.1 and there's no "System" option, but there's "About phone" right on the first Settings screen. However, inside "About phone", there's no "Build number". My phone is a Redmi 6, did they change the Android interface, or Android 8.1 interface is different, or what?