I have a computer with grub and linux installed on the internal hard drive. I also have a USB flash drive with linux that I boot into with grub on the internal hard drive. This causes the USB flash drive to always be /dev/sdb. I would like to know, if I have GRUB on the USB flash drive and boot to it via the BIOS menu, will it be seen as /dev/sda instead of /dev/sdb?
No, it depends on the disk. For new SSD via nvme for instance the disk is called
nvme0n1. And if you have also a SATA disk you can also have a
sda disk which is not the boot drive.
As far as I know nothing prevent the computer from booting on sdb or something else, you can by the way configure the bios to use one disk in particular for booting.