Installing inside Lumia 650 Windows10 (rather than USB key)

I'd like to know as well.

try and tell us then

can you install any other windows programs on Lumia just as they arrive for desktop installation?

I've certainly not seen any indication that Windows 10 Mobile can actually run "Standard" Windows Desktop Software - there is no claim of that capability on Microsoft's own site - so, if the question is "Can PortableApps be installed on Windows 10 Mobile devices?" then I would presume that chances the Platform, or any of the existing apps, actually running successfully are negligible at best. Different processors and screen sizes alone are obviously a limiting factor for running standard desktop software without substantial reprogramming, hence the development of the Microsoft Surface range (and similar devices from other manufacturers) for users who really want/need to run full versions of Windows Desktop Software as "Mobile Applications" and, of course, the Mobile Office Apps which closely resemble the Desktop version of Office, but were specifically reprogrammed to run on devices with 'smaller' screen sizes (10 inches or less).

My understanding is that all Windows software has to be re-programmed as so-called "universal apps" to enable it to run successfully on the mobile version of the Windows OS in addition to the 'desktop' version. Such apps are also only (currently) available via Microsofts App Store and I would doubt Microsoft would willingly enable/permit/support any other form of installation. So unless 'universal' versions of Firefox, LibreOffice etc., etc. are made available....

Windows 10 Phone is not desktop Windows and can not run desktop Windows apps. You can't load Firefox or GIMP or LibreOffice onto it. It can only run what Microsoft calls "Universal Apps" which are apps you can get from the Windows App Store. That's all you can run on Windows Phone 10.