Query Azure Service Management with an Azure AD Member Account to report on tenants the AAD User is federated to as a B2B Guest User. Associated blogpost https://blog.darrenjrobinson.com/azure-ad-user-account-federation-report

AAD User B2B Guest Federation Report.ps1

Import-Module MSAL.PS  
Import-Module AzureADTenantID

# Use the Azure PowerShell Well-Known Client ID
$clientID = "1950a258-227b-4e31-a9cf-717495945fc2"

# Get UserUPN
$userUPN = Read-Host -Prompt "Please enter Azure AD User UPN"
$tenantName = $userUPN.Split("@")[1]
$tenantID = Get-AzureADTenantId -domain $tenantName 
$scopes = "https://management.azure.com/user_impersonation"

# Interactive Flow
$clientApplication = Get-MsalClientApplication -ClientId $clientID -Authority "https://login.microsoftonline.com/$($tenantID)/" -ErrorAction SilentlyContinue
$msalCacheObj = $null 
$msalCacheObj = $clientApplication | Get-MsalAccount -Username $userUPN -ErrorAction SilentlyContinue

if ($msalCacheObj) {
    if ($msalCacheObj.Username -eq $userUPN) {
        # new login for tokens
        $myAccessToken = Get-MsalToken -ClientId $clientID -silent -TenantId $tenantId -Scopes $scopes -LoginHint $userUPN -RedirectUri "http://localhost" -Authority "https://login.microsoftonline.com/$($tenantID)/" -ForceRefresh
    }
}
else {
    # new login for tokens
    $myAccessToken = Get-MsalToken -Interactive -ClientId $clientID -TenantId $tenantID -LoginHint $userUPN -Scopes $scopes -RedirectUri "http://localhost" -Authority "https://login.microsoftonline.com/$($tenantID)/"
}

$myFederatedTenants = $null 
$myFederatedTenants = (Invoke-RestMethod -Headers @{Authorization = "Bearer $($myAccessToken.AccessToken)" } `
        -Uri "https://management.azure.com/tenants?api-version=2020-01-01 " `
        -Method Get).value

$federationTemplate = [pscustomobject][ordered]@{ 
    defaultDomain           = $null 
    customRegisteredDomains = $null  
} 

$outputResult = @()
foreach ($fedTenant in $myFederatedTenants) {
    $fedDetails = $federationTemplate.PsObject.Copy()
    $fedDetails.defaultDomain = $fedTenant.defaultDomain
    $fedDetails.customRegisteredDomains = $fedTenant.domains
    $outputResult += $fedDetails
}

$outputResult