I found these very helpful
- https://courses.cs.washington.edu/courses/cse417/17wi/slides/Graphs3.pdf
- https://www.cs.purdue.edu/homes/jblocki/courses/381_Fall19/slides/Week9.1.pdf (note: the start introduces DFS, the later part explains Tarjan's algorithm)
Note: these pages are also captured in the Wayback Machine.