The Unreal navigation system is based on a Navigation Mesh (Nav Mesh for short). It entails dividing the navigable space into areas – in this case, polygons – which are subdivided into triangles for efficiency. Then, to reach a certain place, each triangle is considered a node of a graph, and if two triangles are adjacent, then their respective nodes are connected. On this graph, you can execute a pathfinding algorithm, such as A* with a Euclidean distance heuristic, or even something more complicated (e.g. variants of A* or systems that take into consideration different costs). This will produce a path among these triangles where the AI character can walk.
In reality, this process is a little bit more complicated, because considering all the triangles as nodes of a giant graph will produce a good result, but it is inefficient, especially...