"...The ISO file cannot be bootable simply by pasting the container onto the USB pen drive..." Meanwhile, I understood (?!?) that and extracted the ISO-file ("container" as you named it) with 7-zip (?!?) to the stick, but it still didn't work. I hope (?!?) that I comprehend it right (I am not a pro + English is not my first language) that you actually explain that in your second paragraph section (2."Legacy BIOS boots the MBR bootsector ..... found inside an ISO.")
Rufus and manual extraction only transfer the file contents inside the ISO. The MBR is either already there and ready, or needs to be created separately. The MBR could already be there, it could be the Volume Boot Record that isn't.
Are you able to just erase the USB pen drive? If so, the simple standard of using Rufus can be done, instead of this highly complicated technical method to avoid erasing it.
If you don't want to erase the USB pen drive, you can do this if it's NTFS.
If the partition is FAT32, this won't work.
Open Command Prompt as administrator
bootsect /nt60 I:
bootsect /mbr I:
Replace I with the letter of the USB pen drive partition.
But reading your third paragraph, section (3."The easiest ..... in Rufus.") I can see that you are talking about a different way, due to the fact that you still point to the complete ISO container, mentioning a second stick (I have only one).
The Rufus method intakes a complete ISO container because Rufus is what extracts the contents inside after formatting the USB pen drive. As Rufus erases the pen drive, the ISO file has to be stored elsewhere while using Rufus. That doesn't need to be a USB pen drive, it can be the internal HDD or anywhere but the USB pen drive chosen as the destination in Rufus.
Does that mean that my extracting way with 7-zip is wrong or isn´t it partly equivalent to your way, you are offering in the fourth paragraph, section ? All the described steps I obviously do on a different computer?!? Confusing!
Extracting the ISO contents isn't wrong. Just that if you extract the contents manually, you also have to write the bootsectors manually. This is the part that needs to be done as well: "then open Command Prompt as administrator and use
bootsect /nt60 I: replacing I with the letter of the letter of the bootable installer partition
" as instructed after the first quote in this message.
Finally, in your fourth paragraph, section (4. "If you don't want to erase....to be formatted.") you offer another (preferable) way, where I obviously have to format the stick to NTFS (how?, I will google that)...
Yes that's only an option if the USB pen drive is already formatted NTFS. If it's formatted FAT32, which is most likely, you'll need to erase it to format NTFS. If you want to erase the pen drive, you could format it NTFS in File Explorer, but if you're not saving the existing data anyway, just use Rufus.
Furthermore, I understood (?!?) that my boot order is not as it should (USB STICK first, but how can I change that in BIOS, see picture "2b.jpg")
From the boot priority menu in the BIOS configuration, use arrow keys to highlight the SanDisk Cruzer Blade, then press F6 to mode it to the highest position, and then press F10 to Save and Exit. From the single time boot choice menu, highlight Removable Devices and press Enter, then the SanDisk Cruzer Blade may appear under that.