Many "illegal aliens" enter the US "legally" through the visitor or worker visa programs. However, many have no intention of returning to their native country when these temporary permits expire. Others enter "illegally" by crossing the US borders with Canada, Mexico, the Pacific coast, and the southeastern coasts in Florida and other states.
Bear in mind that most countries have very long borders, often in the thousands of miles, and it is not practical to watch the whole border all the time, or to fence it off (and even if you did fence it off, it is possible to get through or over a fence if you really want to) so people just sneak in. It is only if you enter at an approved location, such as an airport, that the customs and Immigration officials can check to see if your arrival is legal. And even then, some people used forged documents to get past customs and immigration officials. Some people hide in shipping containers, although that is very risky. So there are lots of ways.
They can stay in that country for 3 monthes before they have to become legal or they have to have an alien card.
One way is jumping from a boat to another boat
they go over the border or swim through
Sneak across
They can, as long as they don't get caught.
The names of the first illegal aliens in the United States are not known. There are millions of illegal immigrants that are in the US, as of 2014.
Please don't equate "illegal aliens = criminals". Anybody can, and sometimes does, commit crimes, not just illegal aliens.
Yes, if they aren't illegal aliens.
23 (America)
12 million
can my illegal spouse travel with me on a airplanne?
Tens of thousands
Humankind won't know until we develop a way of space travel outside of our solar system that doesn't take 10,000 years. ...Or, until the aliens come to us.
Depends on how many illegal aliens... One truck per every illegal alien.
Human traffickers or desperate citizens.
Legal aliens came to the US with the proper paperwork. Illegal aliens came to the US without that paperwork. Some of them came to evade dangers in their own countries or because they had family here. Some came for economic opportunity. Some came to participate in criminal activity.