Old post however here's how I find the download folder using Power Automate Desktop. I Hope it helps someone:
1. Actions > Folder > Get Special Folder - set "Special Folder Name" as "Documents" -> save into variable (%downloadsFolder%)
2. Actions > Text > Replace Text >
set "Text to parse" as %downloadsFolder% ,
set "Text to find" as "Documents",
set "Replace with" as "Downloads"
Then to access a file in the downloads folder use: File path: %downloadsFolder%\FileNameGoesHere.doc