Do you specify the material and set requirements for the qualification of the employees?
I set requirements mostly regarding the quality of the materials used; rarely do I, for example, specify brands. Whether the bidder dares to provide the warranty even when using "Estrich-Achmeds" is their business.
Why do some solid house providers switch to drywall for interior walls?
They usually do so only in pitched roof attics or limited to non-load-bearing interior walls. With a slope, drywall has a labor cost advantage; without a slope, stone (or gypsum board) is often cheaper.
Why do some master craftsman companies no longer participate in some general contractor tenders?
Regarding general contractor tenders, I assume bad experiences—tenders from amateurs are often simply a quality punishment and a nuisance; the reasons for non-participation are different there. With respect to general contractor tenders, I think there will also be a significant difference in non-participation rates depending on whether one is dealing with GCs with proper commercial operations or mailbox company GCs (owner-site manager exclusively with subcontractors).