Allegretti, "Simplicial Sets and van Kampen's Theorem" "(An elementary discussion of the fundamental groupoid of a topological space and the fundamental groupoid of a simplicial set)".
Allegretti, "Simplicial Sets and van Kampen's Theorem": A discussion of the fundamental groupoid of a topological space and the fundamental groupoid of a simplicial set* Animations to introduce fundamental group by Nicolas Delanoue