Hi @atoi ,
Trezor is fully open source and for integration with 3rd party services any app/service developers can use Trezor Connect, which is a platform for easy integration of Trezor, and integrate it into their platform/app.
As for Defi platforms specifically, most of them use integration with Metamask and you can easily pair your Trezor with Metamask (https://wiki.trezor.io/Apps:MetaMask)
Then you can connect straight to these apps and your private keys will still be protected by Trezor.
Regarding supporting coins, it is true that at the moment we are trying to implement what we support directly into Suite and we have no capacity to integrate new coins.
More on this topic here: New coins support
You can follow our roadmap here: Trezor · Suite · Roadmap 🔥 · GitHub