There is no simply method available in SharePoint Online to see the structure of the SharePoint sites.  The script below tries to get some inventory which may help you may be during the migration or may be documentation of SharePoint Online Sites.  The script iterates all sites and sub sites. The script uses Basic Client Side Object Model components.  I suggest you download the latest Components from GitHub.

$Creds = Get-Credential
$Creds.UserName
 
Connect-SPOService -Url https://tenant-Admin.sharepoint.com -Credential $Creds
Connect-MsolService -Credential $Creds
$Users = Get-MsolUser -All
$UnLicensedUsers = Get-MsolUser -UnlicensedUsersOnly
$Users.Count
$UnLicensedUsers.Count
$Users.Count – $UnLicensedUsers.Count
 
Add-Type -Path “C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll”
Add-Type -Path “C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll”
 
$SitesIncludingPersonal = Get-SPOSite -IncludePersonalSite $true -Limit All -Detailed
$SitesIncludingPersonal | Select * | Export-Csv -Path C:\temp\PondSites1.csv
$Sites = Get-SPOSite -Limit All -Detailed
foreach($asite in $Sites)
{
  Set-SPOUser -Site $asite.Url -LoginName $Creds.UserName -IsSiteCollectionAdmin $true -ErrorAction SilentlyContinue
}
$Sites | Export-Csv -Path C:\temp\PondSites.csv -NoClobber -NoTypeInformation
foreach($asite in $Sites)
{
  $ctx = New-Object Microsoft.SharePoint.Client.ClientContext($asite.Url)
  #Authenticate
  $credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Creds.UserName , $Creds.Password)
  $ctx.Credentials = $credentials
 
  #Fetch the users in Site Collection
  $ctx.Load($ctx.Web.Webs)
  $ctx.ExecuteQuery()
  $Count=0
  foreach($aWEb in $ctx.Web.Webs)
  {
    $Count++;
  }
  Write-Host $asite.Url $Count
}
 
$Databases = $null
$Databases = @();
foreach($aSite in $Sites)
{
  $Users = Get-SPOUser -Site $aSite.Url -Limit All | Select * -Verbose
  foreach($User in $Users)
  {
    $DB = New-Object PSObject
    Add-Member -input $DB noteproperty ‘SiteUrl’ $aSite.Url 
    Add-Member -input $DB noteproperty ‘DisplayName’ $User.DisplayName
    Add-Member -input $DB noteproperty ‘LoginName’ $User.LoginName
    Add-Member -input $DB noteproperty ‘IsSiteAdmin’ $User.IsSiteAdmin
    Add-Member -input $DB noteproperty ‘IsGroup’ $User.IsGroup
    $Databases += $DB
  }
}
$UsersOutput = “C:\temp\AllSitesUsers.csv”
$Databases | Export-Csv -Path $UsersOutput -NoTypeInformation -Force
                                                                      
 
$Databases = $null
$Databases = @();
foreach($asite in $Sites)
{
  $Groups = Get-SPOSiteGroup -Site $asite.Url -Limit 100 | Select *
  foreach($Group in $Groups)
  {
    $DB = New-Object PSObject
    Add-Member -input $DB noteproperty ‘SiteUrl’ $aSite.Url 
    Add-Member -input $DB noteproperty ‘DisplayName’ $Group.Title
    Add-Member -input $DB noteproperty ‘LoginName’ $Group.LoginName
    Add-Member -input $DB noteproperty ‘OwnerLoginName’ $Group.OwnerLoginName
    Add-Member -input $DB noteproperty ‘OwnerTitle’ $Group.OwnerTitle
    $RolesString = “”
    foreach($Role in $Group.Roles)
    {
      $RolesString+=$Role
      $RolesString+=”,”
    }
    Add-Member -input $DB noteproperty ‘Roles’ $RolesString
    $Databases += $DB
  }
}
 
$UsersOutput = “C:\temp\AllSiteGroups.csv”
$Databases | Export-Csv -Path $UsersOutput -NoTypeInformation -Force
 
$Databases = $null
$Databases = @();
foreach($asite in $Sites)
{
  $ctx = New-Object Microsoft.SharePoint.Client.ClientContext($asite.Url)
  #Authenticate
  $credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Creds.UserName , $Creds.Password)
  $ctx.Credentials = $credentials
 
  #Fetch the users in Site Collection
  $ctx.Load($ctx.Web.Webs)
  $Lists = $ctx.Web.Lists
  $ctx.Load($Lists)
  $ctx.ExecuteQuery()
  foreach($List in $Lists)
  {
    if($List.Hidden -eq $false)
    {
      if($List.ItemCount -gt 100)
      {
        $DB = New-Object PSObject
        Add-Member -input $DB noteproperty ‘SiteUrl’ $asite.Url 
        Add-Member -input $DB noteproperty ‘Title’ $List.Title
        Add-Member -input $DB noteproperty ‘ListType’ $List.BaseType
        Add-Member -input $DB noteproperty ‘ItemCount’ $List.ItemCount
        $Databases += $DB
        Write-Host $aSite.Url $List.Title $List.ItemCount
      }
      
    }
  }
 
  foreach($aWeb in $ctx.Web.Webs)
  {
    $Lists = $aWeb.Lists
    $ctx.Load($Lists)
    $ctx.ExecuteQuery()
    foreach($List in $Lists)
    {
      if($List.Hidden -eq $false)
      {
        if($List.ItemCount -gt 100)
        {
          $DB = New-Object PSObject
          Add-Member -input $DB noteproperty ‘SiteUrl’ $aWeb.Url 
          Add-Member -input $DB noteproperty ‘Title’ $List.Title
          Add-Member -input $DB noteproperty ‘ListType’ $List.BaseType
          Add-Member -input $DB noteproperty ‘ItemCount’ $List.ItemCount
          $Databases += $DB
          Write-Host $aSite.Url $List.Title $List.ItemCount
        }
      
      }
    }
  }
  Write-Host $asite.Url
}
 
$UsersOutput = “C:\temp\Libraries.csv”
$Databases | Export-Csv -Path $UsersOutput -NoTypeInformation -Force
$Databases | Out-GridView
 
 
$Databases = $null
$Databases = @();
foreach($asite in $Sites)
{
  #$asite = “https://leapthepond.sharepoint.com”
  $ctx = New-Object Microsoft.SharePoint.Client.ClientContext($asite.url)
  #Authenticate
  $credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Creds.UserName , $Creds.Password)
  $ctx.Credentials = $credentials
 
  #Fetch the users in Site Collection
  $ctx.Load($ctx.Web.Webs)
  $Lists = $ctx.Web.Lists
  $ctx.Load($Lists)
  $ctx.ExecuteQuery()
  foreach($List in $Lists)
  {
    if($List.Hidden -eq $false)
    {
      $ctx.Load($List)
      $ctx.ExecuteQuery()
      if($List.WorkflowAssociations.Count -gt 0)
      {
        $DB = New-Object PSObject
        Add-Member -input $DB noteproperty ‘SiteUrl’ $asite.Url 
        Add-Member -input $DB noteproperty ‘Title’ $List.Title
        Add-Member -input $DB noteproperty ‘ListType’ $List.BaseType
        Add-Member -input $DB noteproperty ‘WorkflowsCount’ $List.WorkflowAssociations.Count
        $Databases += $DB
        Write-Host $List.Title $List.ItemCount
      }
      
    }
  }
 
  foreach($aWeb in $ctx.Web.Webs)
  {
    $Lists = $aWeb.Lists
    $ctx.Load($Lists)
    $ctx.ExecuteQuery()
    foreach($List in $Lists)
    {
      if($List.Hidden -eq $false)
      {
        if($List.ItemCount -gt 100)
        {
          $DB = New-Object PSObject
          Add-Member -input $DB noteproperty ‘SiteUrl’ $aWeb.Url 
          Add-Member -input $DB noteproperty ‘Title’ $List.Title
          Add-Member -input $DB noteproperty ‘ListType’ $List.BaseType
          Add-Member -input $DB noteproperty ‘WorkflowsCount’ $List.WorkflowAssociations.Count
          $Databases += $DB
          Write-Host $List.Title $List.ItemCount
        }
      
      }
    }
  }
  Write-Host $asite.Url
}
 
$UsersOutput = “C:\temp\Libraries.csv”
$Databases | Export-Csv -Path $UsersOutput -NoTypeInformation -Force
$Databases | Out-GridView