This question is a bit generic, but what mobile applications exist to control Smart Home devices? I'm interested in generic applications, not in Alexa/Google/Apple/Samsung provided for their eco-systems, but something that can control different types, or protocols, or works with different hubs.
Does such thing even exist?
Maybe some open-source where people can add new protocols?
You can look at home-assistant. It is very generic, integrates with a lot of 3rd party hardware, is open source and very active. It already has matter support, is leading the effort to add matter to their software (and hardware) faster than any other platform that I have seen so far. Supports Zigbee, MQTT, integrates with Google Home, Alexa and even has rolled out their own voice assistant with wake word.
They have a companion app as well that you can install on android / iPhone and control your devices from there.
You can use MQTT Dash to control over mqtt protocol if you want protocol related app, if you are dev/test oriented and you want to test prototypes you've built on your own you could use python on termux if not, then MQTT DASH is your choice. https://play.google.com/store/apps/details?id=net.routix.mqttdash&hl=en&gl=US
