Oppo does not provide official bootloader unlock tools like Xiaomi or Google. You must use unofficial methods.
Install Nova Launcher, Lawnchair, or Niagara Launcher to completely change the look, feel, and customization of your home screen and app drawer. custom rom for oppo reno 6 5g
This assumes: Bootloader unlocked, PC with ADB/fastboot, and downloaded lineage-21.0-*.zip + vbmeta.img + boot.img. Oppo does not provide official bootloader unlock tools
Installing a custom ROM for Oppo Reno 6 5G is not for the faint of heart. MediaTek chipsets are notoriously harder to develop for than Qualcomm Snapdragon due to scarce source code releases. PC with ADB/fastboot
adb reboot bootloader