I daresay it's a bundle. For some reason all the Western World is a target for migrants in the belief they're going to the Land Of Milk & Honey. Booting them out varies between 'target destinations'.
Like the USofA we have a lot of Central Asian 'stowaways' who come in as 'students' who want to work. In the case of the USofA the green card is the object of desire. In the UK we're saddled with going along with what Strasbourg (Court of Human Rights) says. Migrants who try to get in on lorries or through Eurotunnel are told by people traffickers to rip up any ID they carry, thus making it impossible for Immigration Control to send them back. They won't answer questions and they have to be put up in conditions better than the average open prison, although they're not allowed out in case they break parole. Some are given 'tags' to monitor their movements, but somehow they get free and vamoose. Until they commit a crime they're out free to do whatever they want, often it's work as taxi drivers, in Halal abbatoirs (slaughter houses) Halal kitchens at restaurants, deliveries or takeaways.
When Immigration Control eventually catches up with them they still can't send them anywhere. Eastern Europeans from Romania or elsewhere (Romanians sell forged passports to Moldavians or Russians or other non-EU citizens) might come in after committing anti-social crimes at home, but they're a headache to get rid of, like Abu Hamza ('Hook'), who defied the Home Office with the aid of 'shysters' and Legal Aid until we were able to despatch him to the USofA. He came in illegally as well, and started radicalising Asian youth in London. His 'harvest' went to join ISIS in Syria and iraq.