clarification on train, test and val and how to use/implement it
When fitting via model.fit(X_train, y_train,..), will Keras shuffle the
data autonomously?
Yes. shuffle = True is default. So, it basically shuffles every time.

Next, in Keras, you are able to provide the validation set inside the model.fit() method as validation_data=(x_test, y_test) but there is also the possibility to provide e.g. validation_split = 0.2 What is the difference?
The difference is that you can manually provide a validation data set. It's not X_test, Y_test. Rather X_test and Y_test are used for model evaluation model.evaluate() or model.predict(). model.fit is used for the training dataset. When you say validation_set = 0.2, it takes 20% data from training dataset and provides you the validation accuracy and loss.

And after that, the test/prediction set will be taken into account just as model.fit(X_pred, y_pred,..). Right?
You need to use model.predict(X_pred, Y_pred)