There is only one way that can a channel or group be hacked.
They contact the owner of the channel and receive key information from him to gain control of the channel.
They identify themselves as the telegram company.
They send a message to the channel owner asking him to enter a website and enter his phone number and identification code. ( This called “Phishing”)
After receiving the information, they enter the channel control, manage themselves, expel the original owner of the channel, and then completely eliminate and destroy the channel owner’s account.
That’s all. ( Beware of Fake people!!).
How to secure Telegram so they can’t hack it?
I suggest to set 2FA password for your account to make it secure.