Basically everything in the humanities, museums, etc.

Although those jobs tend to be low-paying and insecure as well.

Or they're real lottery jobs that may effectively require volunteer/low-wage dues paying.

But, in any case, internships that don't pay or pay minimally tend to indicate a field that you probably won't earn a lot in anyway.

There are probably exceptions with some summer jobs where the pay may not be a lot but it looks good on a resume in a field that does pay good salaries.

Kids going into medical school will often volunteer in research labs or shadow doctors for free during undergrad. Schools won't consider kids who haven't done this.

There are also paid ways to do this as well like being a medical scribe.

Also if you're poor there much bigger barriers to being a doctor than an volunteering. Like the decade of unpaid schooling. Or remove the need for an undergrad degree for doctor's like the rest of the world.

It is very different from continent to continent and country to country, but the not paid internships I know are for positions that prepare you for tough to get jobs or where the training the intern receives is far more valuable than the work the intern does. Some internships are closer to free schooling in a company with the option to get hired after 3 months. There are bad companies that use internship as cheap or unpaid labor, but that's the abuse of the system, it does not means it is always bad.

I believe film and media. I guess those are only well-paid for a few. A search for "unpaid internships" brought up PR. I'm guessing it would appear more often in industries where networking is much more important and job mobility is lower than in STEM.