A main board (or logic board, motherboard, etc.) will replace the CPU, GPU, RAM, Bluetooth, Wifi, and most of the other components. The SSD is replaceable and not soldered down to the main board. However, that does not mean they won't replace it or reinstall OS X. I've had cases where they replaced the logic board on an old Macbook Pro and also reinstalled OS X for solid reason. Sometimes they like to cover the bases and make sure that the issue never returns. Make sure you back up your data before having it serviced if you haven't already. Good luck!