When it’s summer, what do college students who live in dorms do?

College dorms are only open during the semesters when classes are offered. If a college has no summer term, then the dorms close, and the students go back to their families' homes. Nearly all college dorm leases are for four months (one semester) or ten months (two semester) only. 

If a college has summer classes available, then dorms are available as well, for the students enrolled in summer classes. You would almost certainly have to switch dorm rooms; you aren't able to just stay in the same room as you did in the spring semester. There will be a few days between the Spring term ending and the Summer term beginning, and between the Summer term ending and the fall semester beginning, that the dorms are closed entirely for cleaning and maintenance.  

As an alternative, in colleges in city and suburban settings, there are generally a number of students that live in off-campus housing. If someone needs to stay near campus but doesn't necessarily take classes (for example, an international student who can't afford to fly home), they will often sublet an off-campus apartment for the summer, from a renter who rents the apartment year-round (most non-academic leases are for one year minimum) but who doesn't want to remain there all summer. 

What if an international student doesn't want to fly home, but attends a rural college that has no summer housing? Well, that's the sort of thing one has to consider when determining whether to attend, and which, university overseas. Some will put most of their stuff into storage, and spend the summers traveling or working in a place that provides housing, like a resort or summer camp.  

They go back home. In some places, the dorm becomes a hotel.

Some will stay on campus because they are taking summer classes and others will go back home for the summer 

Most universities and colleges offer summer terms. Not all, but most dorms remain open too. 

During summer, college students usually go back home, just as regularly elementary, middle, and high school students.

Typically they go home, or, if they can afford it, they travel.

Typically, all dorms are closed during the summer. (Or, if they have summer classes, during a portion of the summer.) During that time, custodial and maintenance staff do a deep cleaning, wax the floors, deal with the inevitable mice, touch up paint, check plumbing, and do maintenance as needed.

Sometimes there are students who cannot go home for valid reasons, like international students whose home countries are in civil unrest or war, or who might not be re-admitted to the US if they leave (under the Trump administration). The college may attempt to find them off-campus housing, or may leave a portion of one dorm open for such students, moving them once to clean and maintain that area, so over the summer all dorms are dealt with.

They go home. Most  students have  24 hours to vacate  their dorm room after their last finial exam at the end of the year. Dorms also close down during winter and spring breaks. Student athletes who must stay on campus must make arrangements with housing. International student  can try to make arrangements too if they can not go home.  Most everything is close during the break, so all dining halls can be shut down.

During the summer only students who are enrolled in classes can request and pay to live in a dorm.

I don't know of a single college that doesn't kick out students over the summer.  Many colleges earn extra money by hosting conferences and residential summer schools.

Most go home. Others can pay to stay during the break.