Since the installation is just as important as the window itself, a question about this. What should one pay attention to?
To avoid "Operation successful, patient dead": there are window sellers who send subcontractors to install the windows, who somehow just bash the windows into the house, then the windows are installed and the reveals look pathetic and foam celebrations have taken place. Good installation starts with proper measurement. This inevitably means that you can only derive an offer from the plan and must wait for the actually existing shell construction before ordering.
The company has "Installation according to EnEV2019" stated in the offer! Does that mean they install according to current regulations?
No, that is a nice advertising slogan. There is no Energy Saving Ordinance 2019, as the still valid Energy Saving Ordinance (2016) will be replaced by the Building Energy Act. As far as I know, it is currently still in draft form, I am not familiar with its content – and I doubt that it comments on the procedure of window installation. However, I can imagine that the expected tightening of the Building Energy Act compared to the current Energy Saving Ordinance allows conclusions about the ideal installation plane: for example, in the insulation plane; or, with ETICS, how best to adjoin the windows. My window manufacturer episode was just before the currently valid Energy Saving Ordinance, and as a boss I was not involved with installation, but only with warranty and complaints. So I still know a bit about windows relatively fresh, but I was not born with it.