As others have mentioned, there are two different approaches to this, but since you’re new to SmartThings I just wanted to clarify a bit.
As you’ve noticed, the official features will only support one dimming level per automation, which is pretty annoying. But people can and do set up one automation for each light, all using the same trigger. That will work, it’s just more effort to set up and it can be a lot more effort to keep track of what you’ve done with everything.
The alternative is to use core, which is a community created rules engine, essentially a scripting language for SmartThings. Very powerful and allows for all sorts of sophisticated automations. If you have a technical background, you will probably like it very much. Even if you don’t, there are many community members who will be glad to help you set up any specific rule.
So those are the two basic choices. You use the official features and set up one automation per device all using the same trigger or use core and set up one automation that does everything you want to do.
If you want that trigger to be an Icon in the mobile app rather than a button on a physical device, you will also need to create a virtual device to act as that button. That’s pretty simple, and once you’ve created it it will show up on the list with all your other devices so you can use it as an icon in the mobile app, to trigger a routine, etc. Here’s the FAQ for that: