The starting point for a PowerShell script that loops all sites of a site collection to perform any function. The function(s) could be inserted starting in line 27. The script will write a transcript, the filename could be modified in line 51.

LoopAllSubwebs-EmptyScript.ps1

param (
    [Parameter(Mandatory=$true)]
	[string]$Url,
    [Parameter(Mandatory=$true)]
	[string]$TranscriptPath,
    [Parameter(Mandatory=$true)]
	$Credentials,
	[switch]$IgnoreRoot
)

function DoWeb
{
    param (
	    [Parameter(Mandatory=$true)]
        [string]$Url,
	    [Parameter(Mandatory=$true)]
        $Credentials,
		[switch]$Ignore
    )

    Connect-PnPOnline -Url $Url -Credentials $Credentials

	if ($Ignore.ToBool() -eq $false)
	{
		Write-Host -NoNewline "Doing $Url  "

		# place your code to run here

		Write-Host "Done."
	}

    $web = Get-PnPWeb -Includes Webs

    foreach ($subweb in $web.Webs)
    {
        DoWeb -Url $subweb.Url -Credentials $Credentials 
    }
}


#
# MAIN
#

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

$transcriptExtension = Get-Date -Format yyyyMMdd-HHmmss
$transcriptFile = "$TranscriptPath\LoopAllSubwebs_EmptyScript_Transcript_$transcriptExtension.txt"

Start-Transcript -Path $transcriptFile

Connect-PnPOnline -Url $Url -Credentials $Credentials

if ($IgnoreRoot.ToBool() -eq $true)
{
	DoWeb -Url $Url -Credentials $Credentials -Ignore
}
else
{
	DoWeb -Url $Url -Credentials $Credentials 
}

Write-Host -ForegroundColor Green "Done."

Stop-Transcript