IBM invented the modern PC design, but they sold that business to Lenovo. This should not be a big surprise. Often the only American thing in a computer is the name on the cardboard box it came in. Apple assembles systems in Shanghai and ships them overnight to the US.
If you buy a car from Ford, you expect the frame, engine, transmission, generator, and other parts to come from Ford or at least be built to Ford specifications. You do not expect to be able to put a Ford transmission in a GM car.
In a PC, however, the CPU, memory, disk, CD, power supply, and case are all manufactured to industry standards. You can take a hard disk or memory out of a Dell computer and put it into a system made by HP. The brand names you know are the names of companies that assemble, distribute, and support the computers, not the companies that make the parts.
Industry standards change over time to use technological advances to improve performance. Specific standards are backwards compatible. Modern disks connect to the computer with a “SATA” cable and plug. You can connect a SATA Version 1, 2, or 3 disk to a SATA 1, 2, or 3 mainboard and they will negotiate the highest standard they both can do. However, a Ryzen AMD CPU goes in an AM4 socket and not any older mainboard, and a DDR 3 memory chip cannot be plugged into a DDR 4 mainboard. So you do have to know which standards are flexible and which are not.
This is an international business. The mainboard almost certainly comes from a Taiwan or Chinese company (Asus, DFI, Gigabyte, MSI, …). Disks tend to come from Singapore or Indonesia (Seagate, Western Digital). Memory and LCD displays often come from Korea. The external case and the power supply probably come from China.
You can buy the components from CDW or NewEgg and assemble a computer yourself, but you won’t save any money. The big computer makers buy parts in lots of a thousand, packaged in bulk to save packing and shipping. Nine screws attach the motherboard to the mounts on the case. Four screws attach the disk to the disk bay. Then the cables all plug into sockets. An unskilled worker can be quickly trained to assemble a computer every few minutes. You can learn to do it from scratch in a few hours, so if you are looking for an electronics project to keep you busy for weeks, go build a shortwave radio.
The advanced technology is in the manufacture of the chips, not the final assembly of the finished product. A CPU chip is constructed in a plant that costs billions of dollars. The building is on shock absorbers because the vibration generated by passing trucks would disturb the process. People wear spacesuits not to protect them from the environment, but to protect the chips from flakes of loose skin or the particles we exhale in every breath.
Then the chip is packaged in plastic and shipped out. There is a socket on the mainboard. One corner of the chip has an arrow, and one corner of the socket has an arrow. Drop the chip into the socket while matching the two arrows, look from the side to make sure it is flat, then drop a lever to hold it in place. It is harder to tie a shoelace than to install a CPU chip on a mainboard.
In a laptop, the hard disk, DVD drive, and memory are usually standard, but all the other components are custom designed. In a desktop computer there typically are no non-standard components, although Dell occasionally designs a model with unusual power connectors or an odd attachment between the front panel and the mainboard.