The difference is described by the so called crop factor. The coolpixCoolpix P7700 has a much smaller sensor compared to the EOS, thus it will photograph only a smaller section at the same lens focal length. In this case the sensor is so much smaller that it actually looks like it has a focal length longer than the longer actual focal length of the EOS. You basically have to apply a sensors size in the form of the crop factor to the focal length of the lens (and consequently also to the aperture) in order to arrive at the actual, effective focal length.
Your Coolpix P7700's smaller sensor has a crop factor of about 4.67X, compared to the canon's 1.6X crop factor.
When both are 'normalized' to their 35mm/FF angle of view:
- The 6-42.8mm lens on the Nikon gives a 28-200mm "equivalent" angle of view
- The 18-55mm lens in the Canon 77D gives a 29-88mm "equivalent" angle of view
To get an equivalent angle of view to 42.5mm with your Coolpix P7700, you need a 125mm lens for your 77D.
You have a couple of relatively low cost options for your 77D:
- An EF 55-250mm f/4-5.6 lens for your 77D would give you a 35mm "equivalent" angle of view of 88-400mm, which would be roughly twice the "zoom"¹ (magnification) of your Coolpix P7700 when both lenses are zoomed all the way in and images from both are viewed at the same size.
- An EF 70-300mm f/4-5.6 lens for your 77D would give you a 35mm "equivalent" angle of view of 112-480mm, which is roughly 2.5X the magnification of your Coolpix P7700 when both are viewed at the same size.
There's also the Canon 75-300mm f/4-5.6, but it's not a very good lens.
¹ This is an intentional misuse of the word "zoom" to match the way you use it, which doesn't mean what you seem to think it means.
