There is also decentralized exchange software and infrastructure that can be utilized to build your own exchange.
This is not exactly what I want
I'm investigating the engines of "regular" exchanges.
In practice, there are several exchanges that work on DEX infastructure but operate with a web-fronted interface. Crypto bridge would be a great example. From the user' side, they require no downloads. That's why I like the infastructure bitshares offers for DEX projects.