What happens with shares when they are sold? Where do they go to?

Picture a company. The company owns 20 shares and external shareholders 80 shares.

The stocks are dropping in value, so the shareholders owning the 80 shares sell them.

The company still owns the 20 shares and is now ?owner of 100% of shares?

Where do the 80 shares disappear to? Who bought them if they don't disappear and all the market is on sell sentiment?