Volumes are mounted at bootstrap time through services like the "mount@-usr" service. When enabled, these services are wanted by the "local-fs" target, so that default volumes are all mounted by activating that target.
In their turn they depend from services like the "fsck@-usr" service, which run fsck on the relevant volume before the mount service attempts to mount it.
The "fsck@-usr" service orders itself before the "mount@-usr" service.
Note that the "fsck@" service declares itself as wanted by the "mount@" service, rather than the "mount@" service declaring itself as always wanting the "fsck@" service. This allows the "fsck@" service to be optional, and for boot-time filesystem checking to be structured in another manner should that be desired.