FSF has published a commentary: https://www.gnu.org/licenses/gpl-faq.html#MereAggregation

GPL licenses have allowed so-called "mere aggregation", where separate programs are distributed together. Such programs don't have to be all covered by GPL.

On the other hand, if parts are intimately tied to each other such that they are effectively a single program, GPL applies to the whole.

The FSF commentary explains that the judgment depends both on the mechanisms and the semantics of the co-operation. Technical implementation details don't make programs separate if they are intimately designed to work together: "But if the semantics of the communication are intimate enough, exchanging complex internal data structures, that too could be a basis to consider the two parts as combined into a larger program."