Initially you could just write Android apps. Every OS release broke the standard features more and more, however, and you were pretty much forced into their special programming paradigms, yes. For example, originally swiping emulated the d-pad buttons, but that was removed. For a long time you could pair Bluetooth keyboards, mice, and trackpads, but then they broke that for a long time. It may have been added back recently.

Modern era there is a "Mirror API" that's a very limited web based API for serving "cards" to the device and there's a a native SDK with a few hooks for registering for pre-selected voice commands and showing activities with various amounts of liveliness or graphics capability vs. static cards.

So if you are just publishing cards with standard menu actions you can write server side only in Python and several other languages and just communicate with Google's servers. If you want a more native experience you write in Java using an SDK originally based on Android.