The APR usually comes from trading fees and also the DEX that is trying to attract liquidity will incentivize users by offering their native token as a reward for staking in certain pools. Trading fees are usually not that high, unless the price is really volatile you will see a higher APR for a few days but it is misleading because you won’t earn that percentage in a year, you will only earn it if volatility, demand, and liquidity remain exactly the same.

As for token rewards, the exchanges are basically printing their own money. When they first launch their token, emissions will be high to make their APR look attractive, but this has the side effect of causing hyperinflation so the token’s value will go down and the APR will go down along with it. To combat this, the developers will come up with different mechanisms to make their token deflationary and pump the price back up.

The printing their own money aspect does look like a scam, but to be fair, some newer DEXs have tried to create governance models which give their token lasting value rather than being just a get rich quick shitcoin.