A telescope, or telescopic sight is in essence 2 optical systems fitted back to back.
The front part (the objective lens group) is exactly like a camera lens. It collects incoming light & manipulates it into a cone where there is a point at the end of the cone & it all comes together in a pinpoint. That pinpoint isn't perfect so its actually a very small circle. (Called the circle of least confusion)
At one point in that cone is perfect focus, that's where the reticule sits inside the tube.
The rear (eyepiece lens group) glass collects the image of the front group & the reticule & projects it back towards youe eye.
If you design & build a wide angled front end (like a camera's wide angle lens) it collects image forming light from a wide area & bends (collimates) it into a cone, but with more bending it collects from a wider area.
Here's an illustration I found showing a zoom at wide & tele settings that might help clarify the way it works. This is a camera lens, so imagine a magnifying glass added to the output end on the right hand side, to convert to a telescope.
