Manjaro installed to /dev/sdb writes grub to /dev/sda erasing windows boot record by default

and when you remove this monjaro you don’t have windows boot anymore. ofc grub will not work from live usb. solution is .bin from syslinux. can be found on their site with instructions to dd it.

windows 10 ntfs

You can restore the Windows boot loader with some commands.

What partitions do you have on sdb?

Yes, this may happen when system is BIOS/MBR.

The grub loader is written to the MBR of the primary disk.

If you select the wrong disk - in this case your Windows disk - the previous content of MBR is overwritten. Usually the disk defined as the primary in BIOS will be the disk preselected by the Calamares installer.

It is your task to ensure the correct disk is used for MBR - the installer is a computer program with relies on your input.