Originally Posted by
Rudder Sir
Some detail lacking here. The master FCC is set by crew, normally to the side of the PF, initially by selecting the onside FD on first. Selecting the opposite autopilot flips the master to that side (I.e. if left seat is flying, but engages AP B, right side FCC becomes master). The nominated master FCC is annunciated on the MCP and is obvious to crew.
One detail that must not be overlooked is that the crew has control over which FCC is engaged to perform autopilot / flight director functions, but does not have control over which FCC provides STS and MCAS when flying manually. As stated in other posts, first power-on yields STS and MCAS from the left FCC. Each subsequent air-to-ground cycle without FCC electric power down and back up causes the FCC to be used for STS and MCAS to toggle between the two.