An exchange is the plumbing that brings buyers and sellers together under one rulebook: fixed trading hours, listing standards, mandatory financial disclosure and a continuous auction that turns orders into a single printed price.
The New York Stock Exchange and Nasdaq dominate US listings, with Cboe and IEX competing for order flow. London, Tokyo, Hong Kong, Frankfurt and Shanghai anchor the rest of the world, and each keeps its own hours, so a globally diversified portfolio is always partly asleep.
Being listed does not guarantee you can sell. That depends on liquidity, which varies enormously between a mega cap and a micro cap quoted on the same venue.