NI software (LabWindows CVI, LabView, TestStand, etc.) are not limited to NI hardware.
LabWindows CVI is ANSI C based. Most hardware manufacturer's provide C libraries to interface with their equipment.
LabView comes with hundreds of drivers for instrument from many manufacturers.
Many instruments have a serial or GPIB interface. LabWindows CVI and LabView both have libraries to communicate on these interfaces. The instrument manufacturers provide documentation on commands used for their instruments.