The most famous and to this very date only real DEX is "bisq". It has been started in 2016 and runs uninterrupted ever since. Daily volume is still rather low ~ $500k per day.

Currently developers work on an update version called bisq 2.0. There are plenty of tasks you can grab as a contributor and help make it a thing people have easy access to in the near future. If you are not a developer you can start to become a market maker/trader on bisq which helps with liquidity.

Because Monero dominates bisq, although bisq only offers BTC base pairs the Monero community forked bisq and created its own version called Haveno that is still a work in progress. You can even grab tasks on their github for a small bounty.

Be the change you want to see in the world.

For more information look ath their github:

https://github.com/haveno-dex

https://github.com/bisq-network

Why does it matter?

DEX are the connecting points between the crypto world and the real world.

Other than most CEX they won't go away. They won't rug you. they will keep your transactions private. And most importantly they will enable free trade when no one else will.