Joining the military can lead to citizenship if they're young enough.
Marrying a citizen can also lead to citizenship.
Even without becoming a citizen, their children, if they're born here, will become citizens.
There is a distinction between people who follow immigration protocol and those who don't. I China, even following approved, official protocol, it becomes very difficult. The US on the other hand routinely grants American citizenship to foreigners. China makes it very difficult for foreigners immigrating legally and following protocol to become Chinese citizens.
Do they have the intent (legal or not) to stay permanently?
An immigrant has an intent to settle, an expat doesn't. In America, one could theoretically lose work authorization should he refer or present himself as an immigrant (if he or she is on a legal status that doesn't allow for immigrant intent).
Same thing in Europe. Thousands of EU citizens live and work anywhere in the EU outside their home country but have no intent on permanently staying there. Often for decades. They are very much expats.
