As far as I know, in order to be able to list tokens or coins on a centralized exchange,
we have to spend more funds for the coins to be listed, but there are other ways,
yes thats participating in polls for new tokens or new coins held by the CEX,
especially in good exchanges like Binance, it will be very difficult to enter there
Sometimes money is required, huge sum of money is depending on the standard of exchange to be listed on, exchanges like binance have high price of listing but not in all cases, sometimes community votes can push a project to be listed in this top exchange, other times the nature of the project itself can draw the attention of some of this top exchange, a project that has great qaulity and outstanding ideas or ground breaking ideas usually attract top exchanges because they also want to benefit from the hype of the project.
basically it's all about how much benefit the exchange gonna have if they listed this specific coin, it just always works like that and not only limited to exchange but in real business it's also like that too.
If a project wanted to get listed for free then the project should have a really good quality and big community, otherwise it's money talk. I personally think dex is enough for most of projects out there, but some project really want to get opened up into a whole new flow of capitals to their market.