Comment by M95D

No, it's not (just) Busybox. Quotes from Gentoo: https://wiki.gentoo.org/wiki/Merge-usr

> merge-usr is a script which may be used to migrate a system from the legacy "split-usr" layout to the newer "merged-usr" layout as well as the "sbin merge".

> It is required for systemd ≥255 due to changes upstream, but it remains optional for other init systems.