Best way to transfer files from Windows to Windows via Internet

Since version 1803, Windows 10 has OpenSSH for Windows built-in.

For older versions of Windows, you can install OpenSSH manually.

I have prepared a guide for setting up SSH/SFTP server on Windows using this Microsoft build of OpenSSH.

For FTP, you ca use the FTP server built into the IIS (web server). It's not running by default. Note that you can use the IIS to setup an FTP server even without setting up a website.

When setting up the FTP server in the IIS, make sure you force TLS/SSL encryption (FTPS) and disallow anonymous authentication, for security.

See (my) guide on Installing Secure FTP Server on Windows using IIS.

Once you have the FTP(S) server set up, you can use any FTP client. Windows Explorer itself does support FTP(S). Though note that the Windows built-in command-line ftp.exe client is useless as it does not support the TLS/SSL or a passive mode (so it can hardly connect though firewalls and NATs).

You can use Powershell remoting (since Powershell 5)
There are plenty of examples at https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.management/copy-item
which you can also get by running Help Copy-Item -Examples in Powershell.

This is a simple example of copying a local file to a remote computer.

Example 5: Copy a file to a remote computer

$Session = New-PSSession -ComputerName "Server01" -Credential "Contoso\PattiFul"
Copy-Item "C:\MyRemoteData\test.log" -Destination "D:\MyLocalData\" -FromSession $Session

To day it's possible to use a native scp at Windows 10, you can install it by configuration menu, or by PowerShell, for the last option, here is the code:

Add-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0
Add-WindowsCapability -Online -Name OpenSSH.Client~~~~0.0.1.0

Start-Service sshd
Set-Service -Name sshd -StartupType 'Automatic'
Set-Service -Name ssh-agent -StartupType 'Automatic'
Start-Service ssh-agent

For incoming conections we need the OpenSSH Server, and for outgoing, the client.

Remember, if you have an antivirus software with firewall, create outgoing and incoming rules for ssh at port 22

If you are limited from installing tools, folder sharing in source and using "pushd " and copy in the cmd prompt of destination machine worked for me. It can be scripted also I guess.