I am worried that if you contract separately, there will be problems during the construction process, for example with warranty issues or timing if one suddenly has scheduling problems and the house stands there without windows.
And I am not only worried, but certain: if you put the horse before the cart (by making the killer criterion whether the house manufacturer has windows in the equipment list according to your so-called specialist taste), then you will filter the field with a very thin result.
Why do you think it is different with stone houses?
Because with prefabricated timber frame panel houses you have the problem that the windows should already be installed before transport and installation of the wall panels, the reputable house manufacturers know this and will insist that the windows be delivered to the factory. That throws the entire logistics into turmoil (if they even agree to it) if you take a window manufacturer they don't have in their program. With stone wall panels you can practically neglect this difficulty, and with site-built masonry houses the windows come into the walls as separate elements anyway.