Sample code for writing an AWS Systems Manager Compliance Item.

Write-SSMComplianceItemExample

$managedInstanceId = (Get-Content -Path 'C:\ProgramData\Amazon\SSM\InstanceData\Vault\Store\RegistrationKey' -Raw | ConvertFrom-Json).instanceID
$moduleNames = @(
    'AWSPowerShell',
    'ClipboardText',
    'Convert',
    'PSWindowsUpdate'
)

$complianceItems = [System.Collections.Generic.List[Amazon.SimpleSystemsManagement.Model.ComplianceItemEntry]]::new()
foreach ($moduleName in $moduleNames)
{
    $module = Get-Module -Name $moduleName -ListAvailable | Sort-Object -Property Version -Descending | Select-Object -First 1

    $item = [Amazon.SimpleSystemsManagement.Model.ComplianceItemEntry]::new()
    $item.Id = $module.Version.ToString()
    $item.Severity = [Amazon.SimpleSystemsManagement.ComplianceSeverity]::INFORMATIONAL
    $item.Status = [Amazon.SimpleSystemsManagement.ComplianceStatus]::COMPLIANT
    $item.Title = $moduleName

    $null = $complianceItems.Add($item)
}

$writeSSMComplianceItem = @{
    ComplianceType = 'Custom:PowerShellModule'
    ResourceId = $managedInstanceId
    ResourceType = 'ManagedInstance'
    Item = $complianceItems
    ExecutionSummary_ExecutionTime = Get-Date
    ExecutionSummary_ExecutionType = 'Command'
}
Write-SSMComplianceItem @writeSSMComplianceItem

Write-SSMCompliance posts compliance items to Systems Manager Compliance, and IIRC overwrites based on the ComplianceType field.

This gist demonstrates how to post your own Inventory data.
https://gist.github.com/austoonz/0e8a4ff8c9b26c81d6a2b4ddc9b5f63f