Assuming no false-positives from Windows Defender, you just click through a "scary" SmartScreen warning. Note that signing the exe isn't enough to remove the warnings, you then need to build up reputation for your cert by having enough users click through the warnings.
I think a better question, rather than a point about unsigned exes, was: "on a regular consumer version of Windows, what happens if you run literally any program that was not downloaded from the Microsoft Store?". In which case the answer is "you cannot run it, there is a pop-up directing you to the Microsoft Store, and the only way to find out how to run it is it to google the information which will point you through several layers of system menus to disable nanny mode". This will happen to you for something as common and widespread as downloading Chrome or Firefox. Attempting to disable nanny mode will result in an ominous screen warning you about how bad the thing you're about to do is, and telling you that the action is irreversible.
Microsoft is not as bad as Google and Apple, yet, but they absolutely have the power to be as bad and are flirting with how to accomplish greater and greater control of their platform without triggering too much backlash.
MS has mostly abandoned that approach now. But during Windows 8 days? Yeah. There was a legitimate concern that MS will lock down Windows and try to funnel everything through Microsoft Store, establishing an Apple-style walled garden.
The concern was serious enough that Valve took a defensive posture and started investing into Linux support. Which, at first, largely failed - but eventually resulted in Steam Deck.
For sure, and I'm glad they backed off from it. I'm also glad they did it because of how it pushed Valve into making Steam OS so good. But Microsoft really did want to go down the same path, and I do not trust them not to try it again.
The Surface RT that I tried absolutely was, and everything else I'm seeing to make sure I'm not misremembering shows that Windows RT was still locked down. So you should expand on what you mean.
Windows S Mode shows that Microsoft still thinks this is a good idea, too.
You might be mixing up Windows RT and WinRT. The former was Windows 8 for ARM with the Store as the only software source; the latter was the new set of APIs.
