@Thakur, Rohit Thanks for reaching out.
For a test tenant setup for Intune, you can follow this guide which will help you with step by step process of creating a free Intune tenant :
https://learn.microsoft.com/en-us/mem/intune/fundamentals/free-trial-sign-up
Since you mention that you have an onprem setup and devices are getting synced from on-prem to azure AD, the best state for your devices would be Hybrid Azure AD join, that way you can utilize the benefit of both world. Here is how you can perform Hybrid Azure AD join for the Existing devices :
https://learn.microsoft.com/en-us/windows/client-management/mdm/enroll-a-windows-10-device-automatically-using-group-policy
For new Device you can use Autopilot under Intune to set them up as Hybrid Azure AD join - https://learn.microsoft.com/en-us/mem/autopilot/windows-autopilot-hybrid
Your topology seems to be straight forward, 1 on-prem AD syncing to 1 Azure AD tenant, so you do not have to worry about anything, you can go ahead and setup the devices to Hybrid State as per above scenarios. Feel free to let me know if you have any follow up questions.
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