Hello, @SK,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you are planning to migrate your mail server to Microsoft 365 and would like to use Exchange Online, so you would like to get some documentation on the prerequisites for a hybrid AD environment and Exchange Online.
As you mentioned, there are some conditions that Exchange environments need to meet before setting up a hybrid deployment. For more detailed information, you can refer to this document: Hybrid deployment prerequisites | Microsoft Learn
You mentioned "Hybrid AD Environment" is that a hybrid deployment environment? AD is typically used to synchronize local information to the online machine. In order to better solve your problem, please contact me in your free time if you have any other requests.
Please feel free to contact me if you have any queries. If my reply is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.
Best,
Alex Zhang