Nobody can build a modern computer from scratch. Fabricating integrated circuit chips requires dozens of highly-specialized steps, performed in a series of factories each managed by experts in their own particular step of the process. From locating deposits of silicon of sufficient quality, to mining, refining, purifying, crystalizing, slicing, and etching, each of those steps is done by people who lack the knowledge to perform any of the others.
Assembling a computer from components available to consumers is a trivial matter for those with the needed skills. I know a number of people who can do it, though these days they're going to buy a pre-assembled motherboard, power supply, storage unit (HD or SSD), etc., and assemble them, because that's cheaper than assembling each of those components from their respective components.
Science gives us the knowledge to create technology, which in turn gives us computers and all the other marvels of the modern world. Including the GPS system in every smartphone, that connects to the swarm of satellites which would be impossible on a flat Earth. If you own a smartphone you can download Google Maps, or any of a number of similar apps, and have it show you exactly where on the Earth you are, to a precision of several tens of meters. And all because of science, technology, and satellites.
You can also have it show you the distance (via road or direct flight) between any two points on the Earth, and you can easily verify that these distances are compatible with a round Earth, not a flat one. And if you are a pilot and have an airplane, you can verify these distances.