Select the Telegram QR code type;
Provide a link to the relevant Telegram account or channel;
Click Customize & Download QR;
Create your own code design and click Download QR Code.
The address of the public channel is visible to every Telegram user, it is in the description for it. For the private one, this link is available only to the administrator in the channel settings.
No, the built-in QR code scanner in Telegram is intended only for registering a new device that you use under one account.