Some developers ported "Pixel-like" interfaces based on 7.1.2 to give the device a cleaner, more modern feel.
: Required to flash custom software, which voids warranties. Oppo F1s Firmware Download 7-1-2
: Downloading firmware from unofficial sources can brick your device or expose it to malware. Oppo F1s Firmware Download: Complete Guide to Android 7
: To reach Android 7.1.2, you must use community-built ROMs like LineageOS 14.1 or crDroid : To reach Android 7
No. The F1s Plus has a different chipset (Snapdragon 653 vs MediaTek MT6755). Flashing F1s firmware on Plus will hard brick the device.
. Officially, the device was released with Android 5.1 (Lollipop) and received its final official update to Android 6.0 (Marshmallow)