Side glass is toughened, so it breaks into small pieces rather than cracking. Much of that glass drops down inside the door skin, where it sits on the regulator and the window mechanism.
A proper side window replacement means taking the door trim off, clearing the glass out of the door cavity, checking that the regulator and channels are clear, then fitting the new glass and setting it so it runs and seals correctly.
Experience matters here because there are several different types of side glass. We replace front and rear door glass, fixed quarter glass and the cargo glass fitted to vans and wagons.