How does everyone afford a house?

Correct, although it is 399, or rather 720 for two people.

So at the moment, the interest rates are rather poor... for 30,000 euros you get just over 3,000 euros in interest in 15 years at the current interest rate of my instant access savings account (0.75%). For comparison: prices have increased by about 30% since 1999. Currently – as strange as it may sound – one should really go into debt: the interest is almost entirely offset by inflation. Of course, this only works as long as there is no real estate bubble.
It's also a pity if you have to move out of the house after 10 years because the fixed interest period ends.

Yes, at the moment the interest rates are low, but that was not always the case. For example, we still have a savings home loan contract running with 4.x% interest... we make sure to let it stay until it is really compulsorily terminated. "KdU" were included with me... you can also get a reasonable 70sqm apartment for 2 people in NRW for relatively low rent. And if you keep an eye on offers for groceries etc., you can manage with quite little. Also, as leisure activities besides studying and working, a walk or watching TV together was enough for us. (you don’t really have that much time anyway). Don’t overinsure, don’t go for the most expensive internet connection, consistently choose cheap providers for your mobile phone and no expensive 2-year contracts with a phone, and so on and so forth.

This results in the fact that the second-hand market is in the doldrums - this will change abruptly in 5 years -, the land prices will reach utopian heights; the market for condominiums will be noticeably slowed down. And - due to the internet - increasingly nonsensical advisory talks have to be conducted. Sellers without a conscience currently have Christmas all year round

What do you mean by that? Used properties are expensive too, aren’t they? Condominiums are also expensive, IMHO. The internet leads to nonsensical advisory talks? Dangerous half-knowledge among homebuyers? I’m curious (though possibly off-topic :P

Additional costs 300 euros
Food/Hygiene/Children’s articles 450 euros (112/week)
Office catering 110 euros (22 x 5 euros)
Kindergarten 100 euros
Mobile phone/Internet 100 euros
Fuel 200 euros (2 cars)
Car tax 20 euros (2 cars)
Car insurance 150 euros (2 cars)
Liability insurance 5 euros
Legal protection insurance 25 euros
Household insurance 15 euros
Building insurance 10 euros
Life insurance 10 euros
Occupational disability insurance 70 euros
Dental supplementary insurance 40 euros
Leisure/Clothing/Music lessons/Sports etc. 400 euros
Savings 200 euros
Vacation 200 euros (2 adults, 1 child)

Hm, food, mobile phone/internet is IMHO quite a lot. The thing with the cars you have to decide yourselves, if that’s the case... Insurances: legal protection and household insurance IMHO not necessary; possibly also dental supplementary insurance. Leisure etc. may be a bit high as well, but if the lion’s share goes to things like music lessons, okay. Sports can certainly be very affordable outdoors in nature or in a club.

or a manageable condominium in a good location. For a start.

First a condominium, then a house means buying costs twice (this money is simply gone!). Considering how high the additional purchase costs are, I would personally advise against it.

BTW, interesting topic and also a recurring topic with us (my better half): How do they all do it?! If they do it, why not us too?! My attempts at explanations (combinations possible):
- Very frugal life (cutting own hair; cheap bread, cheap sausage, no cheese etc. etc. and so on)
- Compromises on location (out in the countryside, side costs suppressed)
- Compromises on the property (carport done by own work two years after moving in instead of garage immediately; just one example, cf. also bathroom/kitchen/furnishing/parquet/laminate/etc.)
- Lots of own work (including friends/family)
- Long loan terms/low repayment rates
- Construction started two years ago, when it was still cheaper than today
- Saving phase started 10 years ago, when there were still interest returns
- Inheritance/early inheritance
- Real dual incomes (both!! good education/income; net from 6000 €/month)
- Civil servants with top creditworthiness
- Naivety, unawareness of risks — and then it works out!

What actually still shocks me most are the additional costs. Even with 100% financing, 30,000 to 50,000 euros of "equity" is easily necessary and then gone. So whoever wants to do 80% or even 70% financing needs a chunk of money or has to be modest (mid-terrace house with 120 m² living space on 200 to 250 m² plot).

No cheese? That is quite drastic

The only incidental construction costs that are really gone are the land transfer tax and the notary. Everything else has basically been "invested" – whether it's the building permit, earthworks, or structural analysis.

So first of all, I think you are really approaching the matter correctly. How others can afford to build is beside the point. It doesn't help you if everyone else earns more, lives more frugally, or gets money gifted... or simply had more luck.
With your income, there is already a good foundation. Now the question is what you make of it.
How strong is your desire to own a home? You surely have many wishes and ideas about how your life should look in the coming years. You have roughly calculated that already. Unfortunately, what remains is not (yet) sufficient to reasonably finance the home in addition. And I think that is quite crucial... many finance recklessly. They go into heavy debt and take on a very big risk (low repayment, high interest).
What is not yet included in your calculation... your salary will (hopefully) increase! Significantly more in the first 5-6 years than afterwards. But that also depends on which industry you work in. In the public sector, increases are strictly regulated. In business, they are not.
Therefore, think about what you need for living!? Do not confuse this with what you wish for. Because wishes are usually very costly. Question your standard of living. Do you really need two cars, two households, etc.? These are all costs that make it harder to quickly accumulate the needed equity.
When is the child planned? Perhaps it makes sense first to really push forward in your job and also have the time for it.

Therefore, I would advise you first to create the possibility to move in together. Then adjust your living expenses to your needs, always focusing on saving for the home.
The choice of apartment will already show whether it will be 120 m² or 60-80 m². That already makes a lot of difference in money. You can already get a feel for how much house you actually need. Expectations change, experiences are made, and the world will look different again in a few years.

Assuming you build a house today in the xyz region. Then you can calculate what the land with the house would cost. Assuming it is 400,000 € including all incidental costs. Then your savings goal should be 20-30%, so about 80,000 - 130,000 €. With a potential saving of 2000 € per month, that makes 24,000 €/year ... you could have that saved in 5-6 years. But that also means giving up a larger apartment, generous weekends, expensive vacations, and new cars.

Building a house means binding yourself professionally to a region, which in turn brings less flexibility, which in turn can limit your career (i.e., your income). If you both are satisfied with the situation first and can imagine staying there for the next 5-10 years, it certainly makes sense to settle down domestically.

Long story short... first create the best conditions for your project. Forcibly trying to save on rent is not necessarily the right way.