What happens to CPF contribution from your salary into CPF SA after u reach Basic Retirement Sum? (assuming MA Basic healthcare sum is met)?

Basic Retirement Sum or Full Retirement Sum?

If you mean FRS, the interest stays in SA. Any salary contribution will also increase SA above the FRS. If BHS is full, both MA contribution and interest will flow to SA as well and if at FRS will flow to OA.

There is an allocation ratio based on your age -
https://www.areyouready.sg/YourInfoHub/Pages/Ne...

In any case, if you reached prevailing CPF FRS for your CPF Special account, and/or prevailing CPF BHS for your CPF Medical account, the order of where the money will be topped up will be MA to SA to OA. Meaning to say MA will be have CPF SA's allocation for monies if your CPF SA reached FRS. If both SA and MA are already full then the monies will all be allocated to OA.​​​