Without trying to be exhaustive:

But all I know about Linux is 1: it’s a cantankerous beast that can smell your fear and lack of computer skills and 2: that’s apparently not true any more?

Exactly.

I’m pretty much a fairly casual PC-user, I don’t do much more than play games.

Noted.

Will my ability to play games be significantly affected compared to Windows?

Your queries on which specific games work and don’t work should be answered between the databases of ProtonDB, WineHQ, Lutris and Are We Anti-Cheat Yet?. Note, however, that these are not necessarily exhaustive (even if put together); e.g. after visiting the aforementioned websites, you might think that Roblox can’t be played on Linux. But it’s simply one of the many games that exist in the compatibility blind spots between these databases; as the excellent Sober isn’t accounted for.

Can I mod games as freely and as easily as I do on Windows?

There will definitely be a learning curve to be had. Though, AFAIK, there’s nothing that outright prevents you beyond an initial (and potential) knowledge gap.

If a program has no Linux version, is it unusable, or are there workarounds?

Wine is your best friend in these cases. Or, an alternative. Note that -again- compatibility blind spots in these databases continue to exist; like this significant one.

Can Linux run programs that rely on frameworks like .NET or other Windows-specific libraries?

Again, Wine comes to the rescue.

How do OS updates work in Linux? Is there a “Linux Update” program like what Windows has?

This depends entirely on the so-called Linux distribution you end up installing. Some opt to do updates automatically (perhaps in the background even), while others simply prompt the user whenever updates are available. Yet others expect the user to do them manually. What are your preferences in this regard?

How does digital security work on Linux? Is it more vulnerable due to being open source? Is there integrated antivirus software, or will I have to source that myself?

This is somewhat of a controversial topic thanks to articles like this one. Note that while the article continues to be shared and thus remains ‘popular’, the fact of the matter is that at least some parts of it have become outdated since. Refer to this (more recent) article as an addendum. The gist would be that Linux might be secure enough for your intents and purposes. But this depends entirely on what you intend to use it for. Downloading and executing random files from the dark web is probs a bit much and not something any OS would appreciate. But playing your games through Steam and surfing the internet should be fine unless you’re somehow targeted by a resourceful adversary. If you didn’t worry too much about this on Windows and thus went with the default settings -so no hardening whatsoever-, then popular distros like Fedora should be more than fine for your use case. However, if you require more than that, then you may find solace in the fact that projects like Kicksecure and secureblue do exist. (There’s also Qubes OS, but I’ll assume that’s too hardcore.)

Are GPU drivers reliable on Linux?

In most cases, yeah. Historically, Nvidia used to be a pita. And, frankly, continues to be for some peeps. But it has improved significantly over the last couple of years.

Can Linux (in the case of a misconfiguration or serious failure) potentially damage hardware?

Any bad software (irrespective of platform) can potentially damage hardware. Linux is no different in this regard. Though you shouldn’t have to worry about this unless you intend do some janky stuff.

And also, what distro might be best for me?

As gaming seems high on your list, consider Bazzite.