The term for Senators is six years. Only about one-third of the seats have terms that expire on January 3 of odd years.
Kelly Loeffler was appointed to a seat previously held by Johnny Isakson. The term for that seat does not expire until January 3, 2023. Loeffler will hold that seat until the runoff is decided. Either Loeffler will continue or Raphael Warnock will finish the term.
The seat currently held by David Perdue will be vacant on January 3, 2021. After the runoff is decided, either Perdue will return or Jon Ossoff will get the seat.
For a few days beginning January 3, 2021, Republicans should have 51 Senators with Vice-President Mike Pence as president of the Senate. After the runoffs are decided, say January 6 or soon after, Republicans will have between 50 and 52 seats, inclusive, depending on the results of the elections.
On January 20, 2021, Vice-President-Elect Kamala Harris becomes president of the Senate. The seat she now holds will become vacant and Gov. Gavin Newsom (California) will appoint a replacement to complete the term which ends January 3, 2023. No doubt it will be a Democrat.
What Democrats may do, they cannot do until after January 20, and they will be limited if Republicans retain at least one of the Georgia seats.