OK, I followed your advice from above (Post 11).
I had to use chroot /mnt instead, because manjaro-chroot does not work on btrfs.
Now it is mounted and I tried the next step, doing:
install-grub
EFI variables are not supported on this system.
lsblk: failed to access sysfs directory: sys/dev/block: No such file or directory
WARNING: EFI directory not found! Grub couldn't be installed.
I suppose, that is, because I should mount the esp /boot/EFI additionally to chroot. From this source https://forum.manjaro.org/t/root-tip-recovery-basic-manjaro-linux-recovery/175302 I found the code:
mount -t vfat $esp /mnt/boot/efi
And the reaction was:
mount: /mnt/boot/efi: can't find in /etc/fstab.
OK in the Live-System there is no such a mount point and I will try to mount it as the other subvolumes at /mnt
But this leads to the same reaction. OK, may be, because efi is vfat and therefor cannot be mounted within a ext4 System, which is default when using Live-Stick? But the instructions from the source (URL above) told so.
Just for fun, I tried to change the directory by cd boot and then cd efi and I could get in these directories. I suppose, the Live-Disk does not have boot or efi directories? If so, these directories must be those from the not booting system on nvme1n1.
Also “just for fun” I had a look at the nvme1n1 by using Gparted. And now the black info sign is there again at the fat32 partition. When I click “info” of this partition, I get:
Unable to read the contents of this file system! Because of this some operations may be unavailable. The cause might be a missing software package. The following list of software packages is required for fat32 file system support: dosfstools, mtools.
In the columns of the fat32 partition, there is a simple line which shows, that this partition is neither used nor has unused space. On my laptop, I get this info for fat32 partition, but perhaps this is due to the fact, that this partition is not mounted yet.
Sorry but now I am stuck again at this point.