The Microsoft 365 audit log is the best place to identify user and admin actions (like track admin roles changes).
By default, logs are retained 90 days, except for tenants with Office 365 E5 or Microsoft 365 E5 licenses. But, since summer 2021 (no exact date), the logs are now retained for 365 days.
Note that this only for search with PowerShell Search-UnifiedAuditLog. The web interface are still the 90 days limitation on non E5 tenants.
After some tests on several tenants (M365 Business, Office 365 E3), all now have access to 365 days of history even if the AdminAuditLogAgeLimit is still 90 days. You can find your current age limit with:
Connect-ExchangeOnline (Get-AdminAuditLogConfig).AdminAuditLogAgeLimit 90.00:00:00
I don’t find any documentation with this information, so I don’t know if it’s a non documented feature or a bug…
Search the audit log in the Microsoft 365 compliance center – Microsoft 365 Compliance | Microsoft Docs