Is building a house today not payable within a lifetime?

How big do you actually want to build, how many m² of living space and should or may a second person move in at some point?

Maximum 120 sqm of living space. I plan to live there alone. However, I would plan for 2 people. Additionally, there are 5 birds, which will get a bright corner and an outdoor aviary on the house.

If I were to buy a property, it would have to be an old half-timbered house or a farmstead. But that would be a nightmare in terms of energy efficiency.

Although this is not the topic, have you ever dealt with the renovation of half-timbered houses? I ask because these "ancient" houses, as far as I know, are built quite cleverly in terms of energy consumption. My parents renovated such a house, which is under historic preservation, but that was 30 years ago.

There are also 5 birds that will get a bright corner and an outdoor aviary at the house.

I think that's great. Then you could connect the outside and inside with a sliding glass door or similar, and the birds can be outside or inside as they please. :) I think that's good.

Houses in such villages are thrown out here. Well, it makes a difference whether you are near Frankfurt in a village or in the Rhön in a village where no one wants to go

Don't say that. Here in the Rhön, in villages where even the Rhöners don't want to go, more and more city dwellers are coming, especially from the Frankfurt area.

Seriously, why not a small 100sqm bungalow? If I understood correctly, you are alone, maybe a partner will come later.
A nice living room with a dining area and a manageable kitchen, an 18sqm bedroom, and a bathroom are enough. You theoretically don't even need a guest toilet, it would be a nice bonus.
That will cost you quite differently than building two stories and 120sqm+. Then treat yourself to a staircase to the cold attic (you can insulate it later) and have plenty of storage space there and don’t need a basement. The rest goes in the shed/garage in the garden.

I think that's great. Then you could connect the outside and inside with a glass sliding door or something similar, and the birds can be outside or inside as they please. :) I like that.

Yes, that's exactly what I have in mind. Although a small lift-and-slide door might be too expensive, and depending on the wall thickness, a double window without a mullion will do.
Right nearby there might also be floor-to-ceiling windows with fixed glazing.
What still needs to be considered is the ventilation concept, depending on the season.