Have some doubt on it,
When Bitcoin rises, other coins fall because people are selling their alt coins to go into Bitcoin?
Not necessary at all. Look at the dynamics of bitcoin movement from $ 18,000 to $ 20,000. Many altcoins moved with him, albeit at a lower speed, but still they showed significant growth.
For example Ethereum and Litecoin and other coins. Now, most of the growth in bitcoin is tied to the USDT pair, rather than alcoins.