apparently, President Elect Obama got his briefings before he was official PResident Elect shortly after the election
there is a CIA document that outlines it but I dont feel like going to the CIA website
There are two levels of briefings. There is a slightly lower level briefing that starts soon after the election. It does contain some classified information. Biden has been getting that.
Then there is the exact presidential briefing with TS info that doesn’t start until the transition team is on the govt payroll.