So the entire universe isn’t really 13.8 billion light-years across? It is only the size of a grain of sand?
No. Inflation when things were moving faster than the speed of light is thought to have started when the universe was 10^-36 seconds old and ended at 10^-33 seconds, at which point it was about 1 mm in diameter (per the theory). Since then the universe has been expanding and the speed of light has been the speed limit.