They have lots of experts. Every tire firm has an engineer for each team. But the drivers are the one who choose tires. So you must have really good radar with gravel/ice crew notes that help you to choose the right tires. Also you leave service part sometimes 2 or 3 hours before special stage start and in this few hours it can start to snow. After you leave service park, you can only put tires that you have in your car. That's why drivers sometimes put spike tires in X type. One spike on the front and other spike on the rear on opposite side. Not that easy...