内联注解应该有助于理解其中的逻辑。如果您想学习一些新知识，您应该亲自查看脚本中使用的命令。
此链接https://regex101.com/r/h0SYVz/1应解释以下内容：

$_.Name -replace '(?<=_).+'

$targetFile = 'T:\Tests2022\testscript.txt'

# Enumerate all Directories in Test2022

Get-ChildItem T:\Tests2022\ -Directory | ForEach-Object {
    # if this Subdirectory has the `testscript.txt` in it
    if($_ | Get-ChildItem -Filter *testscript.txt) {
        # go to the next Directory, nothing to do here
        return
    }

    # if this Subdirectory doesn't have the file,
    # extract everything up until the underscore for the folder name
    # and concatenate with `testscript.txt`
    $name = ($_.Name -replace '(?<=_).+') + 'testscript.txt'
    # Join this folder's absolute path with the new file name
    $destination = Join-Path $_.FullName -ChildPath $name
    # now we can copy the `.txt` file into this folder
    Copy-Item $targetFile -Destination $destination
}