The internship does not need to be included in your education, you can choose to do the internship beside your studies. The internship can be done at any time during the year, which means you can also do the internship during the summer. It is also possible to complete the internship within one year of completing your education, as long as you have not applied for your degree certificate.
You can apply and be granted Erasmus exchange several times. However, you can participate in a maximum of a total of twelve months of exchange (study and/or internship) per level of education. That is, twelve months for the bachelor's degree, twelve months for the master's level and twelve months for the PhD level.