You are right, it is not easy. We use general equilibrium models as our basis. Under restrictions like a lockdown, economic activity decreases in certain sectors, such as hotels, restaurants and travel. We then see the repercussions that has in our general equilibrium model. When the opposite happens, meaning when the restrictions in those sectors are lifted, we use the model that has been used and refined on many occasions over the years to see the effect it has on all the other sectors of the economy. That is how we manage to make the forecasts.
Of course, that also requires a healthy dose of professional judgment. We do not rely solely on a model. We make adjustments to account for particular situations, such as whether they apply regionally or nationally.