As /u/azazel1 posted the two main resources are
https://spec.torproject.org/
https://svn-archive.torproject.org/svn/projects/design-paper/tor-design.pdf
Other than that freehaven.net has some great papers, but you'll have to find the relevant ones.
https://www.freehaven.net/anonbib/date.html
https://gitlab.torproject.org