Does Slack actually render any externally-provided HTML? Last I checked, they had full control over all the rendered content, including the snippets that show

Does Slack actually render any externally-provided HTML? Last I checked, they had full control over all the rendered content, including the snippets that show up when you paste, say, a YouTube video. They do use HTML UIs here (e.g. the embedded YouTube player), but is this stuff coming directly from the upstream source or from Slack? I would hope the latter. After all, it seems you can paste a direct link to a video file (e.g. .mp4), and Slack renders its own video player. I never understood why Slack didn't spend their many billions of cash rewriting the desktop app with something like React Native. Even if they do need to render third-party snippets, they could still embed web views for those instances, which should hopefully be in the minority.