Now things get a little complicated……….
You should first read about crop factors
and a thing called “equivalence”.
Equivalence is what we use to compare different camera formats (sensor sizes) on an equivalence basis. We can say that a compact camera is fitted with a '28-120mm lens' but the unspoken word in this description is tit has a 28-120mm “equivalent” lens. We do this as it is an easy It's a way to describe the range of fields-of-view that the lens offers, cancelling out the effect of sensor size by using a common reference point, the lens focal length on THIS particular camera.
A 100mm equivalent lens on a small-sensor camera will gives the same composition and (because that means shooting from the same position) the same perspective as an actual 100mm lens does on a full frame camera, regardless of sensor size, because they are equivalent.
This logic that the idea of 'crop factors' are based upon. The 'Four Thirds' sensor format has a diagonal very close to half that of a 'full frame' sized sensor. And if you calculate the angle-of-view of a 50mm lens on a system with a crop factor of 2, it's the same as for a full frame camera with a 100mm lens.
However, it is not just focal lengths that should be thought of in “equivalence” terms. There is also an 'equivalent' aperture (f stop) value as apertures and sensor sizes interact in more ways than simply a field of view equivalence.
In the film days people did not have to directly compare quality and characteristics across different formats as people did not use vintage lenses on full frame cameras. We just knew that 35mm was better than 110 format, medium format was even better than 35mm and large format was even better still. In the digital era, the significance of (and difference between) sensor sizes meeds to be appreciated, and this is not helped by a poor naming system. I had no idea why a f0.75 lens had exactly the same exposure settings as a f1.6 lens when I swiped them on my camera to take the same image UNTIL I realised one was for a small sensor area and the other for a large format giving them both an equivalence of f0.9!