Bitcoin is the trading pair of practically every other cryptocurrency on the exchanges. A lot of smaller cap cryptocurrencies do not have other trading pairs. What this means is that all the money that goes into those cryptocurrencies and away from them has to go through Bitcoin. At the start of a bull run, all the new money goes into Bitcoin first and from there to the other cryptos, Conversely, when a bear market starts, investors pull their money away from altcoins (= other cryptos) into BTC and from there back to dollars/euros/yen/etc. Bitcoin has the largest market capitalization (market cap = units in circulation x price per unit) of all cryptocurrencies and it is the least susceptible to random fluctuations. That also explains its lower volatility.

The reserve currency for crypto is bitcoin at the moment.  Most recognizable, accepted, traded therefore most valued by the masses.  It as a whole realizes less volatility than the smaller coins or alt coins on the platforms so they are more of a safer haven when volatility in crypto is expected.

BTC is the reserve currency for cryptos as of right now, with a few others trailing in a much more minor effect due to BTC visibility as the generic term for many people in the public right now that don't follow or follow little if at all.  People that are more hardcore followers have their own opinions in ways of hedging themselves in the crypto world but until BTC is no longer the most visible and until more of the general public looks to understand crypto that is the way it's going to be.  Has little to do with what actual BTC does but acts more like an exchange for all things crypto right now as far as market prices go.

Because they are based off of the base value of a bitcoin which is a satoshi. They get their value form how many satoshi they are worth. So when bitcoin goes up so do they.