StandardScaler's mean and standard deviation for real-life data?
It has to be saved somewhere i.e. Database after the training is done.
The saved values should be used on new data and all these steps should work in a loop i.e. when you re-train again the values will be updated and saved again.
e.g. If we see the Keras pre-trained models, it provides the necessary pre-processing function. We can directly use that
from keras.applications.resnet50 import preprocess_input
train_datagen = ImageDataGenerator( preprocessing_function=preprocess_input)