A service needs a server, not just a phone. Do you want to self host, or use the entire hardware installation in someone’s cloud data center?
You can run HA on lighter hardware, or use a VM or docker, but it needs something to host.
When I say “hey Siri open my garage door”, my phone does voice recognition, calls the Apple Home app, which calls my local Home Assistant server, which calls the RATGDO which opens the door.