Exit polls will give prelim results starting about 10am EST. We will get exit poll results all day for each state. The balloting is electronic so pretty quickly processed, which (I think usually) starts long before the real polls closed. Any early results will mostly be absentee ballots -- traditionally they were mostly conservative (elderly & military), but USA has large diaspora of 'Others' (like me) who tend to strongly vote Democrat. Plus some states count absentee votes after the ballot day, each state has own ways to do it.
By the time the polls close in California (7pm PST, 3am GMT), the final results will be pretty clear on POTUS race, but may need double counting & thus delayed results in the swing states.