The methods and algorithms powering advances in modern science, medicine, communications, entertainment, etc. would be impossible to develop, much less run, on something so rudimentary as a TI-99/4A. The applications we harness our technology for have become much more sophisticated, and so too must the technology stacks underpinning them, to the point that no single individual can understand everything. Take something as simple as real time video communication, something we take for granted today. There is no single person in the world who deeply understands every single aspect, from the semiconductor engineering involved in the manufacture of display and image sensors, to the electronics engineering behind the communication to/from the display/sensor, to the signal processing and compression algorithms used to encode the video, to the network protocols used to actually transmit the video, to the operating system kernel's scheduler capable of performing at sufficiently low-latency to run the videochat app.
By analogy, one can understand and construct every component of a mud hut or log cabin, but no single person is capable of understanding, much less constructing, every single component of a modern skyscraper.