Cliquez on App Information in the General section.
Scroll down the page and click on Transfer app.
If all the criterias are meet you will be able to start the transfer process. If not, you will see a list indicating which criteria has not been met yet.
We can see here that the TestFlight Beta Testing criteria is not met yet
If all criterias are met you will be asked for the recipient account Apple ID, and its Team ID.
Below the User Access section, indicate whether you want to give all users on your App Store Connect team access to the app or limit access to just Admin and Finance roles. Once the transfer is complete, you can limit access to specific people within your account.
Below the App Privacy section, if the previous owner has already disclosed what data is collected from the app and how it may be used, you will have the option to either keep existing responses to privacy questions or clear responses and answer the privacy questions yourself. You can view the existing privacy details by clicking View Existing Details.
Read the terms, select “I have read and agree to the agreement presented above,” and click Accept.
Your app is now in the process of being transfered.
It can take up to two business days for the app transfer to complete, during which the app status is Processing App Transfer.
While the app is in this state, the transferor can’t edit the app metadata, pricing and info.