Where to put portable programs in Windows

There is no "official" place to put them. You can put them wherever you like. The only thing to be concerned with is access. Make sure you, and any other accounts that will access the portable apps, have the proper permissions to access and execute the apps.

Personally, I put them into a folder on the root of the hard drive, in something like "utils" as then I know where to find them if necessary. For apps I use across multiple machines, I may install them into Dropbox/Google Drive/OneDrive as to have access to them across all my machines.

C:\Users\<username>\AppData\Local\Programs\ seems to be the proper location.

It's not entirely clear to me what the location would be if the app is to be available to all local users. I'm guessing C:\ProgramData, but I don't know if this directory is "roaming" or local.