what are the things you required to transfer the ownership ( means what tool and which user ) ?

User:

Billing Administrator:

Select  Transfer billing ownership for the subscription that you want to transfer. Enter the email address of a user who's a billing administrator of the account that will be the new owner for the subscription.

Tool:

Azure Account Center can be used.

reference:

https://docs.microsoft.com/en-us/azure/billing/billing-subscription-transfer#transfer-billing- ownership-of-an-azure-subscription