there are many ways where it A cryptocurrency can listed on Exchange. For centralized exchange, they want documents and all others necessary papers for Get listed.even they also analysis that token, is that valid or not. After thay, they announcing listing on their website and social channels. Besides if owner can be a Liquidity Provider, then token can also being trade on decentralised exchange. But low quality exchange Only need money for list, no documents! That's why scam token essily get listed on low quality exchange and start scam people
If you want to get listed on a popular exchange then a lot of research is needed to avoid scam coin trading, so not all top volume altcoins are worth trading on top exchanges right now because they are just hype coins and will disappear soon after a downtrend, but many low reputation exchanges list various altcoins without reviewing the potential details of the altcoin, the exchange only wants to profit from trading fees.