Hey Raja,
in principle, adding a device to FortiManager works as follows:
- you add the device in Device Manager with IP and provide admin credentials
- FortiManager will discover and add the device
- you are then prompted to import the configuration and policies
-> this creates a policy package that mirrors the current FortiGate configuration
-> you can then make policy changes on FortiManager and push them out to FortiGate
-> the first push in this way will delete a lot of unused objects from FortiGate; this is expected behavior
If you make changes on FortiGate, these are automatically synced to FortiManager device database (config backup) but NOT policy package; you would have to import policies manually to reflect those changes.
If you only want to use FortiManager as a location for backups, you can operate it in backup-mode: https://docs.fortinet.com/document/fortimanager/6.0.3/administration-guide/23431/adom-modes
If you are looking for a more thorough guide to FortiManager in general, you can check FortiManager documenation: https://docs.fortinet.com/product/fortimanager/7.0
There are several unaffiliated series on YouTube covering it as well (look for NSE 5/FortiManager or similar), some of which are of decent quality.
