Most people believe that the price of altcoins' is determined by the price of bitcoin and that they tend to follow it's market movements.
I partially disagree with this. I have studied prices of coins and tokens on CMC and the fluctuations of each currency seems to be unique and independent of any other digital currency. Some show strong pumps when BTC goes down and some heavy dumps when BTC is rising.
Undoubtedly, there are market forces and factors which affects most currencies, this is not influenced by bitcoins price, it rather controls bitcoins' price as well as others.
So when trading or investing, make sure you treat each currency differently as they have different utilities.
Bitcoin affects the market because it is the bridge between fiat money and other altcoins. When new investors come to crypto, they will buy Bitcoin firstly, so the increase in Bitcoin price refers that new money has joined the market which is good thing for overall crypto market. That happened in the last bull run, we saw Bitcoin price pumping, then altcoins, but what we see now is pure manipulation because the marketcap is just 1/4 of last December marketcap.