the ++unique++ string is a feature of plone.resource and ensures unique URLs which can easily be cached. I just before talked with @jensens about that - we need that ++unique++ (++webresource++ in Plone 6) because cache-breaking URL search strings (?t=1234) are not allowed with some popular CDNs (right @jensens ?).
The ++production++ is part of the resource merging into production bundles and done in CMFPlone.
See my comment from: Revert "use / for font url to fix production-bundle issues" by erral · Pull Request #168 · plone/plone.staticresources · GitHub
the default.css is created here: Products.CMFPlone/Products/CMFPlone/resources/browser/styles.py at c4014de40e21cd02c089861da0a7b6baf8850ab2 · plone/Products.CMFPlone · GitHub
The++plone++productiontraversal is handled here: Products.CMFPlone/Products/CMFPlone/traversal.py at c4014de40e21cd02c089861da0a7b6baf8850ab2 · plone/Products.CMFPlone · GitHub
And the++unique++traversal here: plone.resource/plone/resource/traversal.py at 2ded97029aca909232940e30ab058da92660edb2 · plone/plone.resource · GitHub
For completness and because I just found it along the way, the++theme++traverser is here plone.app.theming/src/plone/app/theming/traversal.py at e6b0ea42e9078e4ed2365fab6cc656098abe727b · plone/plone.app.theming · GitHubhope that helps!