You have a TCP/IP network. This means that PC A and PC B have an IP address each. PC A knows PC B's address and creates an IP packet for PC B. Then, the packet (Layer 3) becomes an Ethernet frame (Layer 2): PC A gets PC B's MAC address and uses it as the destination L2 address. When the frame arrives at SW1, the switch looks at the destination MAC address and controls (in its MAC table) to which port that address is associated. Then, the switch sends the frame to PC B through that port (forwarding phase). The switch leaves unchanged BOTH the source and the destination MAC addresses inside the frame.