Refugees fleeing from other countries often arrive in waves because they come from the place that is currently experiencing the most trouble in the world. There are times in history where many areas of the world are experiencing trouble concurrently, and we get refugees from multiple places. Often these refugees arrive with very little fiscal resources, and need to learn things like local languages in order to fit into their new community and earn a descent wage. They will congregate together for mutual support, and often in sections of town that have lower rents and consequently are less well maintained. That is what we refer to as ghettos. It is more about circumstance, and not discrimination.
As the refugees integrate into the local community, and begin to earn better incomes they will try and improve their lot. Often they will move out into more affluent neighborhoods and a new generation of refugees from other parts of the world will move into the lower rent district. That is why the ghettos change character over time. Sometimes people will stay in the lower rent district, and use their financial resources to improve them and they become trendy for you well off white people to move in and gentrify them.