you cannot hack fb,youtube, instagram platforms easily. That is very very difficult. They take their security very seriously. But the way to get hold of accounts (like derana) is by fooling the user to reveal his/her credentials. This is called social engineering. So once they have the credentials they can take over the account.
Then regarding the motive of the attacker, There can be few motives. Either they may be looking to get a ransom by holding the account (didn't appear to be the case here) or either they just wanted to find a bigger audience (which seems to be the case here).
These sort of attacks are becoming very common these days. Holding accounts for ransom makes a lot of sense because youtube has become a mode for people to earn money by creating content.
If you wana learn some more just let me know

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