Hello Elisalisa and all.
Maybe it's better I do the answer, because I built these constructions. Gaby and me share this hobby and also the cacti.
In fact, these are not greenhouses but standard plant tables from aluminia baught by a supplier of greenhouses.
On the tables, there is a self-made roof-construction made from aluminia square tube (25mm x 1.5mm) with steel strenghened plastic connectors. The roof construction is topped by PVC-roof slabs from building centre. Around the construction we have strenghened plastic sheets which we can leave down on demand.
The main job is to protect the plants from rain during the vegetation period. In winter, we use it for our hardies.
Viele Grüße
Hans Georg