FortiAnalyzer_7.0_Study_Guide-Online.pdf page 64: To upgrade FortiAnalyzer HA cluster firmware:
1. Log in to each secondary device.
2. Upgrade the firmware of all secondary devices.
3. Wait for the upgrades to complete and verify that all secondary devices joined the cluster.
4. Verify that logs on all secondary devices are synchronized with the primary device.
5. Upgrade the primary device.
https://docs.fortinet.com/document/fortianalyzer/7.2.0/upgrade-guide/262607/upgrading-fortianalyzer-firmware

Regarding ADOMs (Administrative Domains) in FortiAnalyzer, a fabric ADOM is capable of including all device types that FortiAnalyzer supports. This is part of the flexibility offered by ADOMs to manage and report on logs from various devices within a Fortinet security fabric. ADOMs can be enabled to support non-FortiGate devices as well, and the root ADOM in Fabric ADOMs provides visibility into all Security Fabric devices. Additionally, it should be noted that in normal mode, you cannot assign different FortiGate VDOMs to different ADOMs, while in advanced mode, you can, which provides a more granular control over the log data from individual VDOMs.
Reference: FortiAnalyzer 7.4.1 Administration Guide, "ADOMs" and "ADOM device modes" sections.

A FortiAnalyzer system backup includes configurations, report settings, and system information, but it does not include logs from registered devices or database snapshots. Logs are stored separately and are not part of the system configuration backup.