That would be explained in Building and Testing Locally · Wiki · Plasma / Plasma Mobile · GitLab
The same that kde-builder requires: Building KDE software | Developer
One not-so-obvious thing about plasma-* repos is that plasma-workspace is shared between Plasma Mobile and Plasma Desktop, while plasma-desktop is specifically for Plasma Desktop. If it’s needed in Plasma Mobile, then maybe that feature simply needs to be moved to plasma-workspace I believe.
CC @espidev