The photo industry has selected the full frame 35mm camera to be the benchmark. The 35mm film camera has been around for over 100 years. This camera produces an image on 35mm wide photo film. This film was generated for the motion picture industry. It features sprocket holes along both edges which served to steadily transport film smoothly through a movie camera and projector. The sprocket holes prevent the use of the entire 35mm wide film strip. The usable space between sprocket holes is 24mm. The 35mm still camera produced a negative or slide image that measures 36mm wide by 24mm high. The diagonal (corner-to-corner) measure is 43.3mm. The diagonal measure of this image is the key to the question you are asking.
As technology advances, the frame (image) size produced by the camera naturally decreases. This is because ways are discovered that allow smaller cameras with little impairment to image quality. As an example, the next step-down is the APS-C format, it measures 24mm wide by 16mm high with a diagonal of 29mm.
Now lenses for cameras are characterized by their focal length. This is a measurement taken, from a cardinal point called the rear nodal to the focused image when the camera is imaging a far distant object (like a star). This focal length value is generally expressed in millimeters. It technically conveys the power of the lens (size of images of objects). Additionally, the focal length together with the format size permits the angle of view, as seen by the camera, to be computed. If you were to mount a lens with a focal length identical to the diagonal measure of the format, the angle of view will be 53°. If the format is the classic rectangular shape, the horizonal angle of view is likely 45°. Such a lash up is generally termed “normal” for focal length. Shorter focal lengths yield wide-angle views whereas longer focal lengths produce a telephoto (magnified view).
Since the 35mm format is the benchmark, a useful way to make a comparison is by computing what is called the “crop factor” or “magnification factor”. We divide the two formats' diagonal measure to obtain this value. To compare the 35mm full frame with the APS-C we divide 43.3 ÷ 29 =1.5 (rounded).
What does this 1.5 crop factor reveal? If a 50mm is mounted on a 35mm camera, mount a 50 ÷ 1.5 = 35 (rounded) on an APS-C and you will have a lash-up that delivers the same angle of view.
If a 1000mm telephoto is mounted on a 35mm camera, what focal length mounted on an APS-C delivers a comparable angle of view? Answer 1000 ÷ 1.5 = 600 (rounded).
Find out the frame size of your camera, compute the diagonal and divide into 43mm to find your crop / magnification factor.