You have learned lots of technical information in the first three lessons so let's boil it down into its most important parts.
When you turn on the computer what happens? Before the computer is turned on there's no electricity stored in any capacitors in the CPU. That means there's no information in the instruction set, their are no instructions to be processed , there's no data stored. When power is turned on, electricity pours into the cpu and the instruction set (capacitors that tells the computer what to do when instructions are sent to it). So, the CPU is ready for instructions. Where the first instructions are located is in a special chip called the BIOS chip. This chip, similar to the instruction set, is capacitors that are pre-engineered to be open and closed. When power is turned on these capacitors now have ones and zeros stored as a charge in a capacitor (a one) or no charge in a capacitor (a zero). Those instructions (ones and zeros) are sent to the CPU which interprets them and goes to the next step in the start up routine.
The next step is to get the instructions on how the computer can talk to the other parts of the computer (RAM, CD, DVD, keyboard, monitor, etc. This information is stored in a special chip called CMOS. This chip retains it's ones and zeros (charges in capacitors) after the computer is turned off because it has a rechargeable battery that keeps the charges in place when the power is turned off. Why do they do it this way? They can't engineer the chip to work like BIOS because the information is constantly changing. Change out your CD drive for a DVD drive, you must update CMOS so the computer can remember how to communicate with the new device. So, the CMOS chip must be changeable and also hold a charge after the computer is turned off. After BIOS chips has been read then the CMOS instructions are interpreted and the computer starts to talk to the rest of the parts of the computer including RAM and the hard drive.
We still do not have an operating systems installed so the next instructions tells the computer to look for an operating system. Most computers are set to look at the CD/DVD drive first for the operating system rather than the hard drive. This allows you to put an operating system disk in the CD/DVD drive and start the computer that way if you wanted to change the operating system or upgrade it. If there's no operating system disk in the CD/DVD drive then it checks the hard drive for one. That's where it finds Windows, Linux, or whatever operating system you have installed on your computer.
The operating system instructions are copied off the hard drive into Random Access Memory (RAM). All computer instructions and data that we use must be stored in RAM before we can use it. RAM is simply the same type of capacitors that are in the CPU, BIO, and CMOS. Just small devices that either hold a charge (a one) or don't hold a charge (a zero). After the operating system is copied into RAM the instructions are sent into registers in the CPU, those instructions are matched up to the instruction set and then those instructions are accomplished. After the operating system has completed all it's start up instructions, if you want to start another program (MS Word perhaps), that program must be copied (loaded) into RAM and it's instructions are sent to registers which matches them against the instruction set and those instructions are accomplished. If we want to open a document, then that document file must be copied into RAM, then the instructions in MS Word allows us to change the contents of that file in RAM. When we are done, we save the file and the data that was stored in RAM is copied back to the hard drive and the file there is overwritten with the new data file.