Ursa Minor is a constellation in the Northern Sky. It is traditionally important for navigation because of one of its member stars, Polaris, being the north star. Ursa Minor is always visible north of the 18°N and hidden south of 18°S. Polaris is located very close to the North Celestial Pole, and only visible north of the Equator. The visibility of Ursa Minor and Polaris is only consistent if Earth is a sphere.
The Big Dipper is a bright asterism in the northern celestial sphere. It is always visible north of 41°N and hidden south of 41°S. Flat-Earthers noticed that the Big Dipper is visible all year and use the fact to ‘prove’ a flat Earth. In reality, the visibility of Big Dipper depends on the latitude of the observer.
The angle (or altitude) to Polaris approximately corresponds to the latitude of the observer. This fact is observed on every location on Earth where Polaris is visible.
By tracing the path to Polaris from multiple locations on the flat Earth model, the lines will not point to a consistent position of Polaris. The reason is that the Earth is a sphere and the flat Earth model does not represent reality.