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: 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.
Similarly, services like "gbde@-usr" and "geli@-usr" run GBDE and GELI on the relevant volume before both the mount service attempts to mount it and the fsck service attempts to check it.
Often, all of these are created by importing the /etc/fstab table and converting it into native service bundles.