The many mechanics in Fortnite can be bound to any key or button if you go into your settings and change it. But the specific bind used to crouch by default depends on whether you’re playing on PC or with a controller.
If you’re playing on PC, the default keybind for “crouch” is the left CTRL. If you’re playing on an Xbox or PlayStation and using the “Standard” configuration, crouch is bound to LB for Xbox and L1 for PlayStation.
Using the Combat Pro controller configuration, you can start crouching by tapping the right analog stick. Pushing down the analog stick a second time will take you out of your crouched position.
With the Combat Pro configuration, there's no option to hold a button to maintain a crouched position. Instead, the analog stick serves as a sort of toggle function for crouching.
If none of these options fit the mold that you’d personally like to use, you can change your binds by going to your settings.
Keyboard binding can be found under the settings tab represented by four directional arrow keys. Controller settings are under a tab showing a picture of a controller. That should be directly next to your keyboard bindings tab if you’re playing on PC.