how does model.score(X_test,y_test)'s behave?
You don't specify the language or library you're using. 

Assuming it's sci-kit learn in python then model.score automates the prediction of your data using X_test and compares it with Y_test and by default uses the R-squared metric to so (hence don't need to manually derive y_pred).

If you have derived the predictions anyway (e.g. using model.predict(X_test)) then you can use the metric directly e.g. r2_score(y_true, y_pred).