before booting into twrp, take note of the package name of the app u want to convert to system, like whatsapp is com.whatsapp, and google play store is com.android.vending. can check through settings, or since u have magisk now, can check directly in /data with a file explorer with root access. in /data/app, there are folders for each app with the package name + some random lettering, so just see which folder name is for the app u want to convert.
so once u boot into twrp, go to mounts and mount every system and data partition. then go back to the main screen, then go to advanced >> file manager and navigate to /data/app. open the folder of the app u want to convert, then tap on the blue button on the bottom right corner with the icon of a folder with a tick, then select copy folder. u can now navigate to /system/app or /system/priv-app (for important apps like google play services which require more privileges) and tap on the same bottom right corner button again to paste, then slide the slider to confirm. however, i'm not sure if rn8 supports system as root like the rn7 did. if it does, u should see another folder named system-root in the root of your phone, and u should paste the folder in /system-root/app or /system-root/priv-app. after that can just reboot. if u did everything correctly, then the app should now show as a system app, and if u want u can save some space by tapping on "uninstalling updates" (if it wasn't converted properly it will remain as just "uninstall").
be sure to make backups of the app beforehand (possible with titanium backup since u now have root), and bear in mind that such conversions will be overwritten by system updates, which means u will lose the app after miui updates if the app itself is not updated after this conversion. hopefully i made some sense...