The following is a list of exceptionally well-written lecture notes on certain topics in geometric group theory:
Median Algebras
Brian Bowditch
Algebraic properties of groups acting on Median graphs
Anthony Genevois
Lecture notes on GGT
Alex Sisto
CAT(0) cube complexes, median graphs, and cubulating groups
Mark Hagen
What is a hierarchichally hyperbolic space?
https://flab.perso.math.cnrs.fr/preprints/whatis.pdf
What is a cross ratio?
François Labourie