Put these in a directory containing all your repos. Run FetchAll it to get latest remote changes for all repos in sub-directories, Run UpdateMain to update the main branch in all repos with main checked out and without local changes.

FetchAll.ps1

# Get latest remote changes for all repos in sub-directories
Write-Output "Fetching remote repositories..." 

Get-ChildItem -Directory |
	# Has a hidden .git directory (i.e. is a repository)
	Where-Object { ( $_ | Get-ChildItem -Hidden ).Name -contains ".git" } |
	# Has at lease one remote
	Where-Object { Push-Location $_; (git remote).Count -gt 0; Pop-Location } |
	# Fetch remote changes
	ForEach-Object { Push-Location $_; "Checking $_..."; git fetch; Pop-Location; "`t...Done"; }

UpdateMains.ps1

# Update local main branches if HEAD is on main and there are no changes

function UpdateRepo($dir) {

    # Update a branch if clean, on master and behind the remote
    Push-Location $dir

    "Checking $dir"

    $line1 = "On branch master"
    $line2 = "Your branch is behind 'origin/master' by \d+ commits?, and can be fast-forwarded."
    $line3 = "`(use `"git pull`" to update your local branch`)"
    $line5 = "nothing to commit, working tree clean"

    $current = (git branch --show-current)

    if ($current -ne "master")
    {
        "`tSkipping as on branch $current"
    }
    else
    {
        $status = (git status --ahead-behind)

        if ($status.Length -eq 5 -and `
            $status[0] -match $line1 -and `
            $status[1] -match $line2 -and `
            $status[2] -match $line3 -and `
            $status[4] -match $line5)
        {
            "`tPulling"
            git pull
        }
        elseif ($status.Length -gt 3 -and $status[3] -match $line5)
        {
            "`tSkipping as clean"
        }
        else
        {
            "`tSkipping as up to date or to uncommitted or unpushed work"
            $status
        }
    }
    Pop-Location
}

Get-ChildItem -Directory |
	# Has a hidden .git directory (i.e. is a repository)
	Where-Object { ( $_ | Get-ChildItem -Hidden ).Name -contains ".git" } |
	# Has at lease one remote
    Where-Object { Push-Location $_; (git remote).Count -gt 0; Pop-Location } |
    # Is on branch master and strictly behind remote
    ForEach-Object { UpdateRepo $_ }