3GPP 21.900 v20.0.0 — the document's own text
6.4.3.2 Technical realization specifications
Taught in 6. Work Items and Study Items — how the work is authorised (How standards actually get made, in depth).
These cover both the overall architectural and interface specific detailed specifications. The architectural implications and requirements need to be identified at a very early stage, for the purpose of knowing which parts of the standard (and hence of the system) are affected by a WI, and for the purpose of supporting the identification of cross-WI similarities (and hence more overall efficient solutions).
The overall co-ordination of the architectural/system requirements is with a single group as stated above, whilst the ensuing detailed protocol definitions and specifications may be distributed over several groups (according to their scope).