I think it varies and a lot of companies do it differently. I had a part time internship with a software company over the spring semester which "ended" at the end of the semester but that was just a formality to start a new internship contract full-time and with a pay raise for the duration of the summer. Most software companies use internships as "junior-junior" developer positions, I was still writing code that was impactful and used by the company, it was just understood that it was going to take a couple months to get me to a point that I was semi-autonomous.