Here is the last response from the programmer, very useful:
about zip: you can imagine the whole thing like this:
A partition table is created on an SD card and in it a FAT partition for / boot and an EXT partition for / (the main directory for the Linux system, as well as the drive C: under Windows).
Then everything is installed and at the end an image (i.e. a file) of the card is created, which then contains exactly these 3 things (2 partitions with the corresponding table).
Since this file is then forever large (I think 2GiB), we compressed it with xz, because it achieves a very good compression rate and (at least under Linux!) Is a standard format.
Here I agree with you: xz is not so Windows-friendly, we could rather throw this image into a zip, which will not reach the size of xz, but there are more tools for it, or if I remember correctly , it is even integrated into Windows.
Otherwise you see things too simply: the "few files" you see are just this boot partition. There are some very important things to start the system and also the configuration.
However, the whole system resides on the EXT partition, which you cannot access with Windows because Windows cannot do this file system. This is exactly why Windows always asks if it should format the disk.
You can't change that to FAT either, because FAT also cannot file rights, which are very important under Linux.
In short: there is no file system that you can edit under Windows and from which you can boot under Linux. At least not without opening a barrel of other, even greater, vulnerabilities to errors.
Therefore there is the separation with the / boot as FAT: it is only there that even Windows users can do something with it. Without this restriction, we (or Raspbian, that's the basis of it) wouldn't need a separate partition for / boot.
The bottom line is that it is not enough for you to just create the directory opensky and create the files in it, since this is only configuration: If, I suspect, you somehow got the wrong card, the corresponding card will probably be Software in / file system is missing.
And therefore unfortunately you have to go the long way and download the compressed image, decompress it and write it raw on the card. Since Windows has no integration for such things, it is unfortunately quite complex, yes.
About the SD card slot: So I think that a lot of current computers have an SD card slot, just think of all the digital cameras. The problem is more the Micro-SD -> SD adapter. I have no control over what is included in the kit. The kits are all packaged at the shop, so there won't be much change.
And: the documentation itself is not outdated. If you are not getting the wrong card like you are, the card contains exactly the image that you can download from the website.
And the documentation fits this image exactly. The whole thing is already older (around 2018) and you could certainly update the image, but it works on the whole. And since we all do it voluntarily and many of us today have far less time than 2,3,4,5 years ago, you will have to be patient with "beauty OPs".
Don't get it wrong: I don't interpret your problem as a blemish. I really don't know how your card can be wrong, but I also have no insight into the deployment process. As I said: I'm just the type who wrote the software years ago