Since most of the major altcoins move up or down at the same time as bitcoin, aren't they all controlled? How does it work?

I don't really know what you mean with "controlled", but there is definitely no mechanism that makes all the other cryptocurrencies automatically go up or down in price when Bitcoin is also going up or down. In reality though this still is the case most of the times or at least on a grand scale and there are a few reasons for this. First of all a bear or a bull market is always happening withing the whole crypto market and not only for a few projects. Bitcoin is also still by far the leading project in the crypto space. So it is not really surprising, that most of the time Bitcoin is the first project to make a price move and then the other projects are following.
There must be some mechanism. If we notice last few days when bitcoin was going bearish many investors thought maybe bitcoin is heading for big fall again and so many investors sold their assets despite the loss. They thought they could buy from Dip. But the situation is completely different, Bitcoin is now starting to turn bullish again. If you think from this point of view, whales or large investors have held the market hostage to some extent.