Stream quality change from 160p - 1080p?

I have a question and how can you make the quality in the stream so that you can adjust from 160p - 1080p. If you know it, I want to stream to OBS with stream elements and twitch. I would be happy for a friendly and helpful feedback.

Of course you have to render all the resolutions and quality settings you want to render at the same time. Something like that is best implemented by one or more dedicated streaming servers, which receives the actual input signal and converts accordingly.

How exactly do I do that?

There are extra providers who offer you such a service. For livestreams this is not so trivial, if the streaming platform does not already offer that.

Can you recommend me? And why can't I set this with Twitch? That's just a streaming platform

On Twitch that would have to work natively already. Anyhow with some available resolutions. Maybe you need partner status here.
That's why you do not come to Twitch anyway, if that's necessary. You would have to switch to another platform or just to several Twichtaccounts, each for a quality setting.