How are coordinates reduced in Trimble Access?
this is a very wide topic. So without having more details the best thing I can provide is point you to the current Help document.
All help files etc are collected here:Trimble - Transforming the Way the World Works.
Assuming you open General Survey Help 2017.20 you want to scroll down (better jump) to chapter 20. There you'll find an overview of calculations as performed in General Survey.
This should give a good first overview of both GNSS and optical reductions.
Hope this helps
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