What exactly is a smart home and can it be retrofitted?

Some time ago, I saw a "smart home basic package" from a general contractor in NRW. It cost about 6K extra and was based on Loxone. It included roller shutters and a few controllable lamps.

What is different about it if I let the scenarios run automatically or if I control them via app? :)

Automatically running scenarios probably always do the same thing? Like turning on the TV = lowering the blinds. What if today I want to watch outside while watching TV, but tomorrow I prefer a darkened room?

I would not automatically run scenes for my taste, unless they are everyday scenes.
Your example, however, could be implemented with a combination of weather station and timer through logic in such a way that it mostly meets your wishes. And if not, you can always intervene manually.
One should also not forget that the goal cannot be a fully automated house. At some point, in some way, you will always have to intervene.
That is why the term "intelligent" house is such a misleading concept.

Thank you for your messages.

I don’t think a bus system will be an option for me, but when the time comes I will still discuss it with the electrician and ask him to make me an offer.

Which radio solutions would be suitable for me?

I actually don’t find the Opus uninteresting at all.