No [sd-vconsole]?
The initramfs contains a tentative root filesystem, packaged as a cpio archive. Upon booting, it unpacks this archive onto a ramfs, from which it will then do everything needed for setting up the system — e.g. loading drivers, firmware, et al.
This RAM-based root filesystem uses busybox as a toolbox and as a preliminary init system before systemd itself is started. busybox provides its own /bin/sh.