I see the question of rules of origin (eg. how much of a car assembled here has to be sourced here too before it counts as a UK car) is filtering into the public domain at last. See
https://www.bbc.co.uk/news/business-54345882. The British Chambers of Commerce also have a 'dashboard' which includes the question, but the link to it is problematic.
This is a question that affects all our trade deals, not just one with the EU, and it falls into two parts. The first is the origin of components themselves - too many foreign components and the finished product doesn't count as British, so one could argue that this is an opportunity for the UK component industry. The second is how and who certifies the finished product as being British - the typical method being (wait for it) another piece of paper for the nice customs officer abroad. According to the BBC "The original Brexit deal negotiated by former PM Theresa May contained a route to minimise checks on what are known as "rules of origin". That option was removed as part of the revision to the withdrawal agreement a year ago." (Why am I not surprised?)