There are some courses that require a work experience placement, and these will normally be arranged through the university, or sometimes by directly approaching companies they wish to work at. I assume you just mean normal employment to earn some extra money rather than as a part of their degree course requirements?
Well, then they just find a job like anyone else. Could be by approaching companies directly, or by answering an advertisement, or any other way. A lot of students work part-time during the term (evenings and weekends) and then many try to work full-time during their longer summer break. Students have to pay tuition fees and living expenses here (most of them) and they only get small loans towards these from the government so most will have to work as well, unless they are lucky enough that their parents can support them.
At work they are treated exactly the same as any other employee, there are no special conditions or entitlements or anything. They are just normal employees.