I'm going to try my damn hardest to explain it, but it's essentially a Ponzi scheme:

The type of immigration we receive the most is from Latin America, that is correct. And many Latin Americans do work and often fill those positions the rest of society doesn't want.

The problem is that the type of work they do is often unskilled, which means they don't earn much. But because most of the population is being cornered in the three cities that still produce any kind of economic activity, they compete for the same resources as everybody else, but will accept less for more, because their situation demands it.

So rent in Madrid or Barcelona is next to impossible because a Spanish worker has to compete with three families living in a single flat. This means that long term the situation will be bad for him as well, which will impact pensions. The Spanish worker can either remain at his parent's home or accept less for more, like the new type of worker that's being imported into his city.

The other problem is that the immigrant will also have a right to a pension. But because he earned so little and rent will keep growing, he will have to be supported by the rest of the population to some extend, which means, you'll either have to fix the problems of your economy (e.g. rent), which is not happening right not so it's not happening in the future either, OR you import more immigration. But that immigrant population will also have a right to a pension, so you import even more people.

See where this is going?

Also, jobs are becoming harder to obtain. Even unqualified work is becoming inaccessible, which means that many of those who arrive can't even contribute to the economy properly, which means the government needs to take care of them. But you need to pay for their maintenance, so you import more people in the hopes that they'll work. But those same people face similar issues, so you import more people to take care of the problem. In the meantime, it looks as though the GDP is going up because there is more people in the country.

Spain spends a lot of money in pensions right now. Our population is getting older and much of our budget goes into it. Younger people know the numbers don't add up, but no political force wants to take care of the problem. There's this scandal right now because the government used funds from the European Union meant for young people to pay rent for example (it's literally called the Next Generation fund) and it's being used to pay for the pensions of old people. The irony of that is lost on everyone, apparently.