Due to fact, that I can´t give a comment, I will post an answer, that is not sufficient. However, I hope this might give you little more insight. Furthermore, my knowledge mainly builds on german IQ Tests.
1.) Usually this is done by the institution/person etc., that has developed the IQ test. However, in some cases the parameters aren´t updated. So, you should always check for this. Especially in regard of very old IQ Tests and the Flynn effect
2.) First, the type of test-sample usually depends on the population, for which the IQ test is being developed (if you want an IQ test for intelligent people in their 20´s, you should go and try it at different universities). In case of Germany: a lot of IQ Tests usually take samples in different parts of Germany, with different demographic attributes. Usually people get paid for participating. Still, in most cases it is hard to sample a representative sample, due to fact that people with higher social status and intelligence tend to agree more easily in participating (“sadly”, you can´t force someone to take part). So, the aspects you have mentioned are indeed a huge problem, with which usually can´t be dealt perfectly. Sometimes people are told, that they won´t get feedback on their performance or only, if they wish to. As you might guess, this barely counters the problem. Furthermore, huge representative samples are expensive to gather. Therefore, the sad truth is, that in most cases it comes down to representative approximations (at least in Germany). These are the reasons why a lot of (german) IQ Tests are biased upwards (f.e. in the IST-2000R more people have been graduated from high School (="Abitur") than the average; this is even worse for the extended version - to counter that, the subsamples are weighted differently). Just as a little note: IQ tests for students are usually better normed, due to the fact, that they can be accessed more easily.
Long story short: Researchers often try to cope with selection bias, but often can´t really avoid it.
3.) to be honest: I have no knowledge on the international landscape
Just as a little note: You will usually find more detailed information in the specific IQ manual. And: When you are using an IQ test, you should always think of whether or not your test-participant can be compared with the test-sample (you shouldn´t have any problems in comparing IQ performance of a 35 year old participant when the test-sample is representative for ages between 20-30, due to the fact, that the decrease in the average (fluid) intelligence from 20 to 35 is not that huge (as compared to the increase in the younger age, see f.e. Baltes (1997) Figure 3). But then of course, you can only make judgments relative to the (unrepresentative) test-sample, not to the whole population itself. This is also the reason, why i don´t really like statments like "my IQ is ...", because such statements always depend on the test-sample and test itself, that was being used.
An article, that is dealing (along with other aspects of the human development) with the development of fluid and crystallized intelligence across the lifespan: Baltes, P. B. (1997). On the incomplete architecture of human ontogeny: Selection, optimization, and compensation as foundation of developmental theory. American psychologist, 52(4), 366-380.