If you are referring to downloaded STL files they should be in millimeters and the correct scale. However, STL files are unitless. STL file units are not specific to any measurement system. The units in STL files can represent an inch, a millimeter, a centimeter, or any other measurement unit. By default most CAD systems and slicers will use millimeters. It is highly likely if your slicer is set to millimeters then the print will be to the correct scale. The way to verify this is to measure the part model after it has been imported into the slicer. If it is like 10 times too small then the STL file units are probably centimeters. If it is 25 times too small then it is likely inches.
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