Logi Bolt is the receiver for the Logitech MX Keys series (keyboards and mice). The devices (keyboards and mice) also work with Bluetooth. However, the function with Logi Bolt is more stable and far-reaching, e.g. when accessing protected mode or the BIOS.

Are the answers correct? As far as I can see, Bolt is only compatible with the Mx Keys Mini and Mx Keys Business, but not with the "normal" Mx Keys (which normally comes with a Unifying dongle)?

Yes, I use this together with my MX Keys Mini keyboard.

As I can see on the official Logitech website, yes. Logitech MX Keys uses the Bolt receiver.