I did everything like in the guide except I did not generate a new random key and added it to the harddisk but used the passphrase I always used and wrote that into a file.
Passphrase in file works fine, but it must not have newline at the end.
Verify that it works with
cryptsetup open --verbose --key-file yourfile --test-passphrase /dev/yourdevice
How can I see if the
/.root.key is added to the initramfs?
Use one of
lsinitcpio or just unpack it yourself.
gunzip < /boot/initrd.gz | cpio -div -H newc --no-absolute-filenames
This command may be different if e.g. it's not gzip compressed but xz or something else. Adapt as you see fit.
Alternatively, if the initrd drops you to a emergency shell, you can of course also use that to explore the initrd state then.
hexdump -C .root.key # or similar
it seems cryptsetup or whatever is smart enough to automatically try it for all my other HDDs that also use the same passphrase as the SSD
This is a systemd feature, systemd-ask-password --accept-cached (accept cached passwords, i.e. passwords previously entered). Maybe it has found its way into other flavors of initramfs as well. It's a good solution, typing passphrase twice is still somewhat acceptable, typing it 10 times in a row...
Not all distributions use this feature and I do not know if it cooperates with the keyfile approach as outlined in your guide. You'll have to debug that yourself.