The motherboard determines the limits of your computer. The Chipset on the motherboard determines what features the motherboard can support. Like what processors it can run. What RAM it takes and how much RAM it will take. It determines how many other features like SATA3 or USB 3.0, and type of integrated ports and devices the board can support. The Bios lets you setup the boots sequence, which on-board devices are active. What your timing is for your CPU and RAM. Temp warning, fan control and more.
Many schools offer computer assembly classes I suggest if you want to know more you try one.