Magisk 19.4 brings Android 10 and product partition support and adds a new system-as-root implementation
XDA Recognized Developer topjohnwu‘s Magisk started off as a systemless root method and has evolved over the years into a much more diverse and powerful tool beyond just plain root. But even today, if you need root, chances are that the recommended root method for your device mentions installing Magisk for root. The latest update to Magisk comes in the form of Magisk 19.4 release that brings over Android 10 support, support for product partition, and a new system-as-root implementation.
Android supports two types of partition layout: the traditional partitioning scheme (where the ramdisk is present in the /boot partition and mounted as rootfs and system is mounted at /system), and the newer A/B partitioning scheme, where the system is mounted as rootfs. Google has made system-as-root mandatory for devices launching with Android 9 Pie (as part of the changes done for Project Treble), while A/B partitioning continues to remain a recommended but optional feature due to the changes it requires. In Android 10, the root file system is no longer included in the ramdisk and is instead merged into system.
Magisk has supported system-as-root devices ever since the first Google Pixel, but the implementation used had scope for a few issues. With Magisk v19.4, Magisk will actually mount system to / (root) as part of the new system-as-root implementation. This is a big change: /system is no longer a valid mount point, the root directory is no longer rootfs, but system, and the overlay system is also different. This update and the changes allow Magisk to support Android 10 on A/B devices with fully functioning MagiskHide. Module developers can also now properly modify files present in the product partition. Support for A-only devices running Android 10 will come in a future update.