Quite simple: their own industry, opinion research companies.
Opinion research is based on statistics; in order to get a representative view - meaning that the views from the sample you have choosen can be applied to the rest of the population - you have got to choose the sample carefully, not leaving out any kind of age/social stratum/nationality. The holy grail you want to achieve is a so called
random sample. If you don't know what that such a sample is: pulling out 10 colored balls of a bucket full of 100, and predicting the colors of the rest in it with best accuracy possible.
Working correctly to get a proper sample is hard; if you fail to draw a proper sample you got what is being called a statistical bias, meaning that the statements you are meaning cannot applied to the population. A typical bias could be leaving out certain social stratums, not having a real profile in your sample of the population, and so on.
The classic example on what a bias can cause happened in the 1930s in America,
the polling desaster of 1936:
the popular magazine Literary Digest asked over 10 million readers about who they are going to vote for presidency, Landon or Roosevelt, and received 2,4 million answers. Literary Digest predicted a vote of 57% for Landon and 43% for Roosevelt. So many did believe this would come true.
There was another newcomer to the industry back then, Gallup. Gallup only interviewed around 50.000 people, and predicted the real outcome of 62% for Roosevelt and 38% for Landon. What happened? Quite simple: selection and non response bias. Literary Digest sent questions to people who had phone numbers back then, and asked them to return their answers. So they asked only the middle class, but entirely left out the lower social stratums, and many people of the 10 million they wrote to just didn't answer. Gallup in return manufactured his sample far more carefully, didn't leave out the lower income classes/poor and therefore was able to get far more accurate results using much less people.
This thing is the classical example of bias, how it ruins representativity and lectured to every student of statistics around the globe in their first course.
Coming back to Yougov and their main error: they are doing their public surveys online only. So this means you have got to sign up on their web site, answer polls and earn money. They are not using any other technic which is being used by classical opionion research institutions, like phone calls, asking people directly on the street, and so on - just the internet. So they always potentially do suffer a big, selectional bias by default regardless the question they do research.
By using just the internet with this approach you do have a really heavy bias, because you are unable to reach out to all the different social stratums you need to be able to hear to get a proper sample, and therefore proper results. This is why I distrust Yougov, because this simply cannot work for many questions if not all the people you need to ask are on the internet/internet affine. Many old people for example just won't care about such stuff in the internet by my experience, but you would need to ask them directly. They are therefore
weighing their samples, but this is always another big, big source for errors, because at the end of the day it is always better to have a proper random sample than to recalculate your biased one based mostly on your experience. And this possible bias is rooted in the way Yougov works, it won't go away - ever, therefore weighing their samples is for them a must. This is why I distrust them and prefer studies being done by proper, established opinion research companies over theirs any time.
Selection Bias in UK Polling (Part 2): Internet Polling
https://www.displayr.com/what-is-selection-bias/
Why opinion pollsters failed to predict overall majority for David Cameron
The only real reasons why companies like Yougov, Civey and others are having customers is because they are really cheap compared to their much older competitors; if I would ever need to do serious market research, I would never trust anything Yougov/Civey and so on might come up with.
And most of the press articles where Yougov&Co. are being praised are being rewritten press notices by Yougov, where the authors don't even have the slighest grasp about the fundamental workings of statistics/opinion research and just are taking for granted what Yougov provides for free.