Paying extras is done in a handful of different ways. If they arent union they're probably getting minimum wage for the day, or close to it. They'll also be given some food, quality depends on set but nonunion workers will get the worst food offered 9/10 times. Union extras will get paid a tad bit more, and if they work enough hours throughout the year they'll get benefits. They do of course have to put some money into the union, but the pay still ends up being more than nonunion. They also tend to get a better lunch. Sometimes companies will put out ads for volunteers, finding people who think it'll be fun to spend a day on set day dreaming of being noticed by the director. They wont get paid, and will share the same food as the nonunion extras. Featured extras get paid more as well, and theres some things that'll get you paid more regardless of if you're union or nonunion, such as letting your car be used in the filming. You can put this on a resume if you wish, but if you're trying to get jobs acting as anything but an extra you're better off listing theatre experience and student films. 
Source: I study theatre and have worked as an extra and spent time talking to many union and nonunion extras. Fun fact! At every set there will be at least one person who runs around telling extras "there's no such thing as a small role, only a small actor!" If you are an extra you are a small role. Do not try to make it big by being an extra, it will not happen. You can do it to get some extra cash, see what being on a set is like, and connecting with other actors, but you will NOT be noticed by the big time director and offered a real role in the film. At best they'll ask you to say one or two lines