Curious about MEXC meme tokens

Ok so I’m just curious about how the meme tokens work with this site . Let’s say I look on etherscan and the holders area. You will see let’s say mexc and uniswap top Let’s say from 420 quadrillion tokens the top holder in that will have 10 quadrillion tokens. That is the top whale . My question is - is mexc holding those tokens like a housing unit or are they selling against the people? Why do I see on the history of the graph it goes way up and down? Are they buying more from people and then selling???? Isn’t uniswap decentralized? Why do they go up and down as well? I’m just curious how the holding works for mexc. They hold so much but is this the basic fundamental I guess of the qty total. Then they become the sorta drug dealer ? Or are they working against the people on all these meme tokens?