Are IQ tests "beatable"? I have been watching a lot of Jordan Peterson's videos on youtube recently. One of his themes is, that IQ is among the best studied metrics, with high predictability, validity and is generally believed to be a very real thing in psychology. He also mentions, that IQ can not be improved or trained.

My question is, whether I could improve my IQ (by that I mean the test score, not general intelligence), by simply studying for it, trying to learn about the different types of questions and how they are generally solved and by simply doing a bunch of tests and memorizing how to solve questions.

I have done a few IQ tests online and at the beginning, I was very confused about everything, like from which direction to read the boxes and stuff like that. After doing a couple and seeing how it is done and reading explanations about the different questions I feel, that I am doing a lot better now.

Similar to that, I was on a job interview once, where they asked riddles, presumably in order to assess IQ, without doing an IQ test.

I knew some of the riddles and simply answered some of them by sheer memory.

Isn't it possible to increase your test score of an IQ test by simply studying? And if you can, what makes the tests really valid? And finally, if IQ was such a high predictor of intelligence, wouldn't most job interviews be geared towards assessing your IQ in a legal manner?
Short answer:
Yes, but it does not matter    

Long answer:
Each question has some inner mechanism that you either know or you don't. Usually you should know how to do it, but you don't know, that you should do it that way. Once you know the right method, next puzzle you would solve much faster - most IQ tests are like this. If you don't know the mechanism, it could be very hard to solve - IQ tests should be like this.
For example this one - once you know, that there are 4 squares all the time, next puzzle of this kind will be much easier. Most puzzles use Subtraction and Addition - once you try this two things, you will solve lots of IQ tests. You are expected to know this methods, and that it will be used in test.
Problem with IQ tests is insufficient number of respondents - since it needs lots of people to create norm.
For example I created test with possible score 0-100, and 50% of people reached 80 points. When you take the test and had 80 score, your IQ should be 100. But when (because of better education for example) most scores begin to be above 80, norm will change and for 100IQ you would have to score 82 or more.     

So yes, you can study for it, but when more and more people do it (and knows about using subtraction, 4 squares all the time and so), norm will change and tests would be harder for everyone anyway, since there will be always exactly 50% of people that could reach at least 100IQ. ( = take some online tests, since most people do it, otherwise it would put you into disadvantage.)    

Sources:
Flynn effect  - raise in IQ due to education and also health (notice not genes).
More about norm
Tests commonly used - in it you can hide math operations without using numbers.