How, using grub-mkconfig or good easy tool else, do we have/make new /boot/grub/grub.cfg for USB flash drive (so it'd be mounted on root of USB drive device) containing booting list only a/some booting(s) from that USB drive bootable partitions
You must be looking for something like this: https://github.com/Jimmy-Z/grub-iso-boot
There are also grub payloads in e.g. coreboot that search for configs
An easier methoud would be Ventoy, for instance.
Thanks to you, I just discovered wentoy which is a great tool!