There is plenty of technical documentation available for Varnish. The kind of documentation you are looking for will most likely depend on the version of Varnish you are using.

Varnish Cache documentation

An overview of all Varnish Cache documentation The reference manual for Varnish Cache 6.0 LTS

Varnish Cache 6.0 LTS is the long-term supported version of Varnish and is the version that we recommend. It is maintained by Varnish Software and receives frequent updates, including backported features.

The Varnish Cache community does two releases per year, which are considered fresh releases. These releases are primarily featured-based and do not guarantee backward compatibility.

Varnish Enterprise documentation

Documentation for all Varnish Enterprise tools Documentation for Varnish Cache Plus

Varnish Enterprise is the commercial version of Varnish and the name refers to a collection tools that make up the Varnish Enterprise solution. This includes Varnish Cache Plus, which is the internal name of the commercial version of the varnishd program.