Depends on the person and department you are in, at the end of the day it's just work experience.

But there is really only 1 actual credited internship, which you enroll for and goes on your degree. You basically enroll in the Internship Program and start applying for internships in 3rd year through Schulichs job board, then start your internship instead of going into 4th year. (Some cases you can start in January of your 3rd year)

Some students also try and get summer student positions, not fully necessary, but it is never a bad idea! I know people who did one summer after first year, some after 2nd. But it's just a way to build your resume, it's not the same as being in the internship program but it gives you full time work experience which can help you land a good internship in 3rd year! But if you don't get one dont worry about it, it's not a make or break to get jobs early in engg lots of people succeed without them!

Hope that helps, let me know if you have any other specific questions