Mostly, the airlines outsource this job, which means that they engage ground handling agencies authorized by airport authorities. Actually, it is not economical for the airlines to do the recruiting on their own and make them their own staff because there are many expenses to be borne with in this way such as free food, free transport, concessional or free passes to employee family. So what they do is that they entrust the job to a private company and it take all the intermediary process and expenses out.