A lot of what you're describing isn't related to IQ or IQ tests. What I think you may be referring to are some IQ test scams from facebook. Not exactly sure.

First of all, while I personally can't definitely judge whether IQ is a perfectly accurate measure of intelligence or not, it is one of the best documented and fundamental parts of psychology. It's covered in great detail in any psychology textbook or class.

There are different types of intelligence, meaning five people with the exact same IQ score may or may not have similar capabilities. Some are great at 'logic puzzles', while others excel at more literary puzzles such as debates. Fundamentally pretty much the same concept, but for one reason or another there's a divide there. It does not appear to be directly influenced by personal background or upbringing.

(at least the tests that have no cultural bits, the ones that include cultures also have some testing of your memory based largely on your country's school curriculum. more on why that's a problem later)

IQ tests are supposed to be designed in a way that is accessible to everyone, regardless of ethnicity or culture. That's why they usually consist of very simple concepts, such as general shapes. A square is a square, whether you're from Europe or Africa, and doesn't require any significant knowledge to be distinguished from a circle. If a test relies on any outside knowledge (e.g., you need to know what a cat is vs what a dog is) - it's not an IQ test, or it's a very poor one.

what happens when during their schooling years they have to focus on surviving, instead of learning and nurturing their skills? well, they score lower, of course.

The core issue here is not about 'nurturing skills', because as countless studies have shown, it does not appear at all possible to raise your IQ (i.e. become more intelligent). All attempts at training people to get more intelligent either brought no results, or turned out to be false.

What's happening in the situation you described is mostly access to healthy food, healthcare, etc. The biggest predictors of IQ are general health and nourishment. This is basically why poor people score lower. A brain can only utilize all of its capabilities when it's supplied with sufficient nutrients and is not significantly impaired by sickness.

Of course, as many different traits, IQ can and is being used for eugenics. Basically what you said about poorer groups. It's nothing inherently to do with IQ itself, but the result is the same.

Also, as already mentioned by some commenters, Mensa is just a degenerate group of soft, wannabe Nazis.