How to run a model stored as a .Rda file?
I suppose you come from Python. R is quite different as it does not require to save to external files your model, you just need to have it in your workspace. Once you have a model object, you just have to use the predict function and specify the unseen dataset. Here's an example of a tree model:

model&lt;-tree(Y~X1+X2, data=dataset) #estimate of the tree model
predictions&lt;-predict(model, newdata=unseen_dataset) #prediction on new data


Be sure variable names between datasets correspond.