"IINA is the modern video player for macOS."

I can scarcely imagine a descriptor more empty of meaning than "modern", in this context.

It's mostly just clean UI that I like. And that it's in Swift, makes it easier to contribute than C.

Am I blinkered in that I expect people contributing to a media player to know C, or do you mean in terms of working with the walls of apple's garden (haven't touched an apple product for five years now so I've never used swift)

In addition it seems to be just an UI over mpv, which is written in C.

Qt is only used for linux and windows. AFAIK UI for Mac will be written with Cocoa or something like that

VLC is C++

Did you watch the presentation? He stressed that VLC is only C at its core. Of course whenever they interface with Qt they call into C++, maybe also in some other plugins.

What “UI” does a video player even need? Is the pause button prettier or something?

I'd pay for a Windows version of IINA. Or at least any video player with focus on effective UX, not on a checklist of features and options.

VLC doesn't spark joy for me.

I second IINA. I love its design and UX. Feels like a great native mac app.

Nevertheless, it's good to keep VLC handy too. Sometimes things work better in VLC, and sometimes on IINA -- in terms of being supported or not, bugs in playback or just plain CPU usage.