For individual users, there’s a free pricing model, but Sketch (yearly subscription) and Adobe XD (monthly) are both are subscription as well, so I’d say we’ll just have to all get used to recurring payments. If there’s not one like that, let me know, I’m sure that people here would like to weigh the benefits of all of those options.

You can export the source file:

Though, I’ve never done so, so I’m not sure what that experience is like.

They have a desktop app… and yes, it does work without an internet connection. Here’s me, just a second ago:

It integrates with Framer, which I haven’t tried yet. It’s a short list, true, but I’m thinking they’ll add more in the future.

Hope that helps!