I will try to answer it in short because the list of factors is long like: the first one is everyone in the market has different opinions  and ideology of BTC and ALTs. They think of BTC as the king while the ALTs as Minister. Whenever the price of BTC will increase it means the sentiments of market will change. For example if people have invested in BTC when the price was around $25k then the dominance of BTC will increase which put 6 or 7 different possibilities on the last like if high BTC (d) means lower prices of ALTs, etc.

So, when the BTC . D will be high the last might be lower and when the price of BTC will high holders and traders will try to book profit and dominance might decrease and once it will decrease the price could increase. So the sentiments of market and Profit/loss ratio also depends.

Sometimes, investors are looking for opportunities that could earn them huge profits amid the high risk included so they do for ALTs instead of BTC. Which means BTC was not their first choice due to less volatility and ALTs was their first choice which ill increase the ALTs dominance this te prices of ALTs will increase.

The reason is due to Bitcoin dominance in the market, but since you are new, you might not be able to notice it.

Back then, before the emergence of stablecoins. Most centralized and decentralised exchange had Bitcoin as the base asset in a trading pair. Asset prices would be calculated in sats or BTC. If BTC is at 30K and the price of ETH/BTC was 0.01 BTC (300 USD) an increase in Bitcoin price from 30K to 40K means the price of ETH/BTC will now be 400 USD minus it moving even from 0.01 BTC

The same applies if the Bitcoin price drops.

Today, there are many stablecoin trading pairs, but the altcoin-Bitcoin trading pairs still exist and with Bitcooin's dominance, it will affect other coin prices when it moves.

Now my bro consider how we look at USDT whenever we check price of Bitcoin... although USDT is a stable coin in a way it almost wants to be determined by Bitcoin, it increase or decrease. Again most of these other crypto currencies are always priced against Bitcoin...what I mean is that they're measured in BTC....just like BTC is the measuring instrument of this other currencies...this is one among many reasons why they tend to fluctuate as Bitcoin price also fluctuates
Summarily, Bitcoin is a measuring instrument of the other cryptocurrencies!

In terms of the increase in the price of bitcoin, it seems that it does affect all prices. Yes, it's true if you look at the USD exchange rate. But if it's exchange rate among crypto it can stay the same

Let's try to simulate a bit
1btc = 100 eth
1btc = 1000 usd
1eth = 100 usd

If the price of BTC increases 2x against the dollar
1btc = 100 eth
1btc = 2000 usd
1eth = 200 usd

If you look at the simulation above, the price of btc to eth remains the same. But because the demand for btc is high, the price goes up, so indirectly the price of eth also goes up.

Maybe there is so little explanation, so when the price of bitcoin goes up like crazy and touches ath, then indirectly almost all altcoin prices will also go up, especially altcoins that have btc pairs on the crypto exchange.

This is basically due to market dominance, bitcoin for some time now has been dominating the entire crypto market.

Market dominance is simply calculated by the entire market capitalization of a coin, bitcoin has had and still have the highest market cap in crypto, and as such it's market dominance is pretty high, it's currently around 49.9 percent, this means that in the entire market cap of the whole crypto market, bitcoin alone has 49.9 percent, with a dominance that high, it's very easy for other coins to depend or kind of depend on the price of bitcoin for the to determine their own prices..

We wil begin to see a difference, where every coin could solely determine it's own price when the market dominance of bitcoin comes down to atleast, 25 percent or lower.

At first we hear more about bitcoin then other altcoins in the market, be it self research or just fomo behavior according to someone's information, like a tree I see btc as the root and altcoins as branches and leaves. So the root must be healthy, the new branches and leaves can grow, the money flow is like water and nutrients to absorb into that tree, when we look at the fluctuations of the market we always see the beginning of the growth bitcoin before altcoins can follow suit.

The largest of all cryptocurrencies, think of Bitcoin as the big brother. The innovator, the king of the hill, if you will, is the one who started it all. Its younger brothers, the altcoins, take after Bitcoin in terms of behaviour. Since they hold Bitcoin in high regard, when Bitcoin succeeds, they take inspiration from it and do likewise. They sense the pessimism when Bitcoin declines and follow that path as well.

What causes Bitcoin to fluctuate, then? Supply and demand is an elementary economics principle. 21 million Bitcoins are the maximum number that can ever exist. The supply component is that. However, demand increases as more people become aware of Bitcoin and desire to own some of it. Also, prices rise as demand outpaces supply. Value decreases when there is a decrease in demand or when fear sets in.

So thats a brief summary of the link between Bitcoin and altcoins as well as the impact of supply and demand on Bitcoin's price. There are ups and downs, but its always enjoyable to watch a reality TV show!

Bitcoin dominance in the market is one of the reasons altcoin prices follow Bitcoin price movements.
In addition to trading with stable coins, many exchanges trade Bitcoin with altcoins such as BTC/ETH and BTC/BNB.
When the price of Bitcoin rises, the price of altcoins can also rise following the price of Bitcoin.

Investors and traders who are experienced with the crypto world realise that the ultimate goal in investment and trading activities is to get as much Bitcoin ownership as possible so altcoins are used as an alternative to getting Bitcoin.

maybe we are the same in knowing bitcoin, but I used to know meme coin, which I got from bounties and airdrops when I first got to know crypto, it lasted 2-3 months. I just understood that btc is the parent of crypto itself where every time the price of btc goes up, even only 10-40% has a huge effect on the value of altcoins that have exchanges to btc
so in conclusion, in my opinion, bitcoin drives the cryptocurrency market because bitcoin's market capitalization is bigger than other follower coins like the altcoin