Short answers, pulled from the story.
The ENIAC machine became operational in 1946 and could be run by a single person. This mode of operation predated batch programming or time-sharing systems that connected multiple users to mainframe computers.
Federico Faggin created the Intel 4004 microprocessor in 1971 using silicon-gate MOS integrated circuit technology he developed at Fairchild Semiconductor. Widespread availability of these chips from the mid-1970s onwards made computers affordable enough for small businesses and individuals.
Micro Instrumentation and Telemetry Systems introduced the Altair 8800 in 1974 which is widely recognized as the spark that ignited the microcomputer revolution. It was based on the 8-bit Intel 8080 Microprocessor and Microsoft's founding product was Altair BASIC.
Commodore announced the PET computer in January 1977 making it the first successfully mass-marketed personal computer. The first 100 units shipped to waiting customers in mid-October 1977.
IBM introduced its first PC model 5150 on the 12th of August 1981 operating with an Intel 8088 chip running at 5 megahertz or later 16 MHz. The system could use up to 1 megabyte of memory and came with VisiCalc spreadsheet client and EasyWriter word processor.
One billion personal computers had been sold worldwide from the mid-1970s up to 2002. By late 2022 the market stabilized after demand cooled down with global PC shipments during the fourth quarter of 2024 growing 1.8% year-over-year reaching 68.9 million units.