Short answer:
It depends, somewhat important, but will not be really significant or a game changer for most applicants.
Long answer:
You probably had a long list of ECs to get into medical school, and even the most uninvolved students will still have some ECs in medical school, however these will vary greatly from person to person. Generally speaking, most ECs don't matter, but you should have something as listing literally nothing would just look weird. But there is no real way to compare or contrast ECs between applicants, or to weight them like they do in med school admissions. Because of this, programs are not typically screening or using ECs when determining who they will interview for residency. Now, once you get to an interview, many interviewers will bring up your ECs for one of three reasons 1) it sounds really interesting and they want to hear more 2) they have/had a similar EC and want to talk about it 3) It's a good ice breaker along with hobbies as a low stakes conversation starter. So while ECs rarely will be a deciding factor or push you above better applicants, they are something that may allow for better performance in an interview and make you more memorable. But don't do an EC because you think someone else will care, because they won't. Do it because it's interesting to you and you could see yourself talking enthusiastically about it in person later on.
The one caveat are the 99th percentile ECs that are so rare and so impressive that no one, or very very few people in the country could or would do. The kind of jaw dropping things that make you say "wow, how in the world could a 20-something-year-old medical student do this while still crushing medical school". These types of things will benefit you, but 99% of ECs will not fall into this category.
