Cashing out coins

I have a question if someone makes let’s say 3 million with a mean coin let’s say Pepe. What determines that they can cash out that money and how. For example does the market cap determines they can just sell for 3 million or does the volume determines that.

I always been curious because I always wanted to know how do people just cash out 5 to 7 million dollars do they sell all at once or do they cash out slowly. If they cash out slowly what if the coin dumps within that time can anybody explain that to me simply please