I have a Sony RX 10 IV. I used maximum zoom, 600mm, but when editing in Lightroom in the details it gives the focal length of 220mm.
Can anyone explain, please?
The sensor in the Sony RX10 iv is a one-inch sensor. The "600mm" is actually the "full-frame equivalent" of the 220mm lens you really have. In other words, your 220mm lens produces on your sensor the same framing as a 600mm lens on a bigger (24x36mm) sensor.
If you had a true 600mm lens,
The thing is, your camera doesn't have a 600mm focal length. Sony is lying to you. The Sony RX10 IV has a 8.8-220mm lens. Sony lists it in the specifications here. But they've done a very naughty thing by marketing the camera aggressively using incorrect focal lengths.
Focal length is a physical property of a lens. Combined with a sensor of a given size, the angle of view can be determined. But there are lots of different lenses and lots of different sensor sizes, and it can get a bit confusing for some people when equating things, so there is this convention where people talk about equivalent focal length of a lens - that is, what focal length lens would produce the same angle of view if used on a 35mm film camera (because people were used to equating a focal length and an angle of view from the days when 35mm film was king).
When your camera is set to maximum zoom (220mm), it captures a photo with the same angle of view that a 35mm film camera would if it had a 600mm lens mounted (because the sensor in your camera is smaller than a frame of 35mm film). So the lens is said to have an equivalent focal length of 600mm, and that's what Sony has labelled the camera body with. Lightroom is behaving properly and reporting the real focal length.
It's customary to specify the range of a compact camera as "35mm equivalent". Your sensor is smaller than that of a "full-frame" 35mm camera (which should have a 36mm×24mm sensor) by a factor of 2.7 I think. That means that to get photographs with the same framing (outline), you only need 1/2.7 of the actual focal length. 600mm/2.7 is about 220mm.
For a compact camera (non-removable lens), the specifications in display (unless given as a zoom "factor") and on the side of the lens barrel will be in full-frame equivalent, as will be the advertisements. The script on the front of the lens will specify the actual focal length.
Here is how it looks on a Sony DSC-R1 compact: You can see that the lens is marked on its front with 14.3–71.5 (the true focal length) while the zoom control is marked "Equiv.135" (full-frame equivalent) and goes from 24mm (not depicted) to 120mm. So the "crop factor" here, the fraction this camera's sensor is compared to a full-frame one, is 120/71.5, about 1.67.
Detachable lenses will get marketed with the actual focal length (printed on its front) since the sensor size they are used with could theoretically vary a bit. This usually also holds for "kit lenses" sold together with a camera body.