I will keep this simple - data flows can continue freely for up to 6 months following the expiry of the transition period. Hopefully, within this time, the UK will secure an adequacy decision and the free flow of data can continue forevermore (or at least until the adequacy decision is reviewed!).
For those who are interested, the agreement reached between the UK and the EU is here- see page 406 for the interim provisions relating to data transfers.
The agreed trade deal allows for the continued free flow of personal data from the EU to the UK for a maximum of six months after the transition period expires