Buttons are made a couple of ways- a shank button will have a loop of plastic, wood, metal or something that creates a bit of space for fabric when a garment is closed. A flat button has two or more holes- when applying (or, commonly, reapplying) a flat button to a garment, a shank must be created. This is accomplished by, after sewing in and out of the all button holes 2 times, winding the thread around the stitches at the base, creating a bit of a stand before tying off.
Most ready to wear clothing comes with extra buttons in a little baggy, or sew on the inside of the garment. If there are no extra buttons and you find yourself button shopping- please remember that buttons have their very own sizing, noted as L for ligne or French for line. Measuring diameter, 40 lignes is equal to one 1 inch.
Pictured above, buttons from my private collection.