What I've done is I put the APK to the root of internal storage.
Open your terminal emulator (I'm using Termux) enter "su" to give root permission, then enter "systemize" to launch the script.
Then you'll need to select the 3rd option which should be "systemize given APK".
Next you'll need to enter the path which should be "/sdcard/OnePlusCamera_v6.2.22.apk" and select to install to "/system/priv-app/"
After the process is done, just restart your phone.
This works. Now, how to update the camera app? There's a new version on apkmirror (6.2.36), however, when I try to install it in the same way, after selecting to install to /system/priv-app/, it says that 'APK exists'. How to remove the com.oneplus.camera folder from that directory?