The initial pandemic models are hypotheticals, the folks who study these things run various scenarios with their models in theory all the time. When something hits, they TRY and match the best model they have in terms of what is thought to be known about the new disease.
They presume no inherent immunity to the new disease for the population.
Then they use what limited information is available to estimate how contagious it may be. This one is dicey.
Then they use the same to try and estimate mortality figures.
From there, it's a pretty simple model. The models presume nothing can be done to combat it, no mitigation, no isolation, no face masks, no nothing, at least initially. This spits out what COULD happen, emphasis on COULD, worst case scenario, with a wide range in the estimate, as we have seen.
The big problem with COVID is that latency period wherein a person can be contagious and not know it for 2-3-4-5-6 days.