Mold in the apartment

Hello first of all, I am new here. What I am writing about is certainly an old topic because affected tenants believe they are doing nothing wrong. The house was built around 1914-1920, a 3-unit house, the middle apartment has mold. It is clear that it comes from not ventilating or insufficient ventilation, this can be observed. The tenants say, for example, the house has cracks, probably called kerb cracks, so rain goes through and penetrates the inner wall, according to this statement. However, the wall is about 43 cm thick and probably has an air layer. Assuming it would rain through the outer wall, the water would run down in the air layer, according to my layman’s theory, because how should water run "horizontally" from the outer wall, through the air layer, and through the inner wall. The existence of an air layer is suspected by consulted experts, such as a painter, a civil engineer, a building restorer, all professionals. If there should be no air layer, how can the masonry be about 43 cm thick and moisture penetrate through this thick masonry? As mentioned at the beginning, there is actually only one explanation, it is not ventilated (properly). I would like to hear or read opinions from this forum, thank you in advance.

Have the experts ever measured the moisture in the walls or looked at the matter on site?

Yes, inspected and measurements were taken, water pipes are not leaking, there is no moisture inside the house that could be caused by water supply (e.g., washing machine) or water drainage, all negative.