Copy list items between sites, list must already exist. Target list is added to, not cleared first. Uses PowerShell PnP

Copy-ListItemsBetweenSites.ps1

$sourceSite = 'https://mydomain.sharepoint.com/sites/mysourcesite'
$targetSite = 'https://mydomain.sharepoint.com/sites/mytargetsite'
$listsToCopy = @('List1', 'List2')


# install Powershell patterns and practices (PnP) module, if not already installed
if ($null -eq (Get-InstalledModule SharePointPnPPowerShellOnline -ErrorAction SilentlyContinue)) {
    install-module SharePointPnPPowerShellOnline -AllowClobber -SkipPublisherCheck -Scope CurrentUser
}

# connect to source site
$source = Connect-PnPOnline -Url $sourceSite -UseWebLogin -ReturnConnection
if (-not (Get-PnPContext)) { 
    Write-Output "Can't connect to source $($sourceSite)"
    return 
}

# connect to target site
$target = Connect-PnPOnline -Url $targetSite -UseWebLogin -ReturnConnection
if (-not (Get-PnPContext)) { 
    Write-Output "Can't connect to destination $($targetSite)"
    return 
}

Write-Output "Copying from $($source.Url) to $($target.Url)"

# copy lists
foreach ($list in $listsToCopy) {
    Write-Output "Fetching list: $list"

    # get all the fields except hidden, readonly fields, or the contenttype or attachments
    $fields = Get-PnPField -List $list | 
        Where-Object {-not ($_.Hidden -or $_.ReadOnlyField)} | 
        Where-Object {-not ($_.InternalName -eq 'ContentType' -or $_.InternalName -eq 'Attachments')
    }

    # keep track of items done for progress 
    $i = 1

    # connect to source - page 5000 items at a time
    $items = Get-PnPListItem -List $list -Connection $source -PageSize 5000 -ScriptBlock {

        Param($items) $items.Context.ExecuteQuery() 
        $totalCount = $items.Count

        foreach ($item in $items) {

            # create a hash table of the form: @{"Title" = $item["Title"]}
            $fieldsToCopy = @{}
            foreach ($field in $fields) {
                $fieldname = $field[0].InternalName
                $fieldsToCopy += @{$($fieldname) = $($item[$fieldname])}
            }

            Write-Progress -Activity "Copying list $list" -PercentComplete (($i / $totalCount) * 100) -CurrentOperation "Item # $($i)"

            # add the item to the target site
            Add-PnPListItem -List $list -Values $fieldsToCopy -Connection $target
            $i++

        }
    }
}