Upload

If I select a folder and press the Upload button, will WinSCP upload only new/updated content, or will it upload the whole folder (again).

By default, WinSCP will upload everything.

You can change that by checking New and updated files only in transfer settings:
https://winscp.net/eng/docs/ui_transfer_custom
That makes the upload more like local to remote synchronization.
https://winscp.net/eng/docs/ui_synchronize