Android 10 also introduces a new dynamic partitioning system. Instead of having a dedicated system partition and a dedicated vendor partition, etc. Android 10 uses a super partition. I like to think of the super partition as a partition that contains a bunch of smaller partitions. One of the side effects of this dynamic partition system is that Google has chosen to use a form of the ext4 file system that is for all intents and purposes, read-only. This choice means that even if you wanted to, you can't easily mount and modify the system partition. We haven't really discussed this with other developers yet, but it may impact your ability to do things like install Gapps. In addition, the dynamic partition model means that eventually, we should probably provide you, the user, some GUI driven tools in TWRP to allow you to manage the dynamic partitions that are on the super partition.