Hi
The easiest explanation about Google Play System Updates is here:
https://www.android.com/android-10/#q-security-updatesThis functionality is rolled out by Google .
"Security Patches" (as you are used to) are rolled out by ASUS, baked into the firmware. They are not replacing each other.
Generally we have supported security patches for a minimum of 2yrs from launch, and security patches are aimed at a bi-monthly schedule - schedules however are floating and every now and then it can look a little bit different.