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

A fundamental question to the OP. How far along is the construction already?

When I read the first post, it is mainly about a remotely controllable house. However, that is not a smart home for me. It only becomes smart when many sensors are used and a corresponding programming automates many processes. Whether you want this is something you have to decide for yourself and also be willing to spend the money for it. I think you should get involved in the planning early on and not only when the shell is already built. For this, a good partner and your own involvement are also important.

Apple HomeKit could represent all of this if you pay attention to HomeKit compatibility when selecting roller shutter drives. It probably also works with Alexa, but I have no experience with that.

But that's exactly what you shouldn't do... Why would you want to give control over your own house/electricity to a third party (or even many)?

Back to the OP's question. What is "smart" about that?

Absolutely nothing. It's a zoo of systems that somehow more or less work together with a lot of screen choreography and incantations of the big apple. And once the warranty/support has expired, the toy gradually ends up in the trash,

But that is exactly what you shouldn’t do… Why would you want to give a third party (or even many) control over your own house/electrics? Not a chance, absolutely nothing. It’s a zoo of systems that somehow work together half-heartedly with a lot of dancing around the screen and ghost summoning of the Big Apple. And after the warranty/support expires, the toy ends up in the trash bit by bit,

I wouldn’t phrase it quite so bluntly. But yes, there are dozens of systems and you have to ask yourself what the desired performance brings you. "Smart" is nonsense. Comfort (whether KNX at its best, or Alexa) should be decided individually.

You have to make a choice. Rotary dial, push-button phone or touchscreen.

Already with the reliable detection of presence/absence, most consumer-grade systems fail. Not to mention the detection of whether the present person is authorized, and I won't even start talking about the recognition of activities and conditions—here these systems are far behind, or have not even come up with the idea that this functionality is required.

One has to choose. Rotary dial, push-button phone or touchscreen.

... or voice control. ;) I think the mentioned requirements are not far-fetched, and nowadays there are really dozens of systems that can do that. If you plan everything from the start anyway, I would of course recommend choosing a system that covers all points (well). But you can even use a separate system for each area as long as you do not require interoperability.

Thank you for the many responses.

I don't think I need a "real" smart home.

I wanted to do without, for example, motion sensors because I have a large dog who would probably trigger them constantly.

What do you think of the Weberhaus smart home, which is based on EnOcean technology?

I find the EnOcean technology interesting if there is no power supply at that point. For the blinds, I was thinking of flush-mounted switches, which are then automatically supplied with power.

What is important to me is that everything still works the old-fashioned way by pressing a button but also works with rules and small automations.

Apparently, my demands are not that high :)