Loop all subwebs of a site collection (root site). Needs the PowerShell PnP extensions for SharePoint Online.

LoopSubwebs.ps1

param (
    [string]$Url,
    $Credentials
)

function LoopWebs
{
    param (
        $WebCollection
    )

    foreach ($web in $WebCollection)
    {
        $url = $web.Url

        $subwebConnection = Connect-PnPOnline -Url $url -Credentials $Credentials -ReturnConnection

        # do handling for this web
        Write-Host $url

        $web = Get-PnPWeb -Includes Webs -Connection $subwebConnection

        LoopWebs -WebCollection $web.Webs
    }
}

if ($Credentials -eq $null)
{
    $Credentials = Get-Credential
}

$rootConnection = Connect-PnPOnline -Url $Url -Credentials $Credentials -ReturnConnection

$web = Get-PnPWeb -Includes Webs -Connection $rootConnection

$url = $web.Url

# do handling for this web
Write-Host $url

LoopWebs -WebCollection $web.Webs