If I have an existing photo (in library) how do work with it in Capture

Capture collects a series of services that allow you to convert a proper item into something else.

This is how on my iphone the app looks like, as I open it

As you can see, as it opens on "materials" the camera is activated. I am in a dark room so all you see is a dark screen, but the camera is on. Big white button on the bottom center is what you would use to take a picture.

Now, if I continue to type, shapes and all other options in  Capture, the bottom layout won't change.

If you look closely, there are two icons on the right of the click button, the big round one at the bottom. The first one is used to turn the camera, the second one allows you to open a picture in your library. The icons are universal so you should recongize them steroetypically.

Now what you do want to do with your pictures in Capture, only you know, I don't really understand what "adjustments" mean in this context, but if you just want to work on an existing picture, that is a way to go that applies to all options included in Capture.

Otherwise you can use the universal icon to open the photo gallery on your phone/tablet

If you only want to edit a picture, enhance colors, crop, adjust shadows, contrast, add filters and the likes, I'd recommend you download Photoshop Express.

+1 if you are interested in Photo adjustments, then you would want to use Lightroom CC. Although, if you are simply trying to use the photo which is available on the phone and use it in Capture. There are a few options -

1. If you are using iOS - From the photo gallery on the phone you can select the photo and Open-in the photo in Capture. it will directly launch Capture app, right from the gallery with photo pre-loaded.

2. On Android, you can share the photo from gallery and select Capture as the App. It will directly launch Capture app, right from the gallery with photo pre-loaded.
3. If you already have Capture launched, and if you are in Camera/Library, click on the image icon in the bottom corner. It will open a bottom sheet in which please click on "Camera Roll" to access photos on the phone.

If you are trying to use an image from CC Library, instead of "Camera Roll" please select "Creative Cloud" and it will open asset browser where the second tab is Library.

Hope above helps and happy to help further if you run into issues.

I think you are trying to use the wrong mobile application.  

Adobe Capture is used as a vector converter to turn photos into colour themes, patterns, type, materials, brushes and shapes it is not used to make adjustments to your photos for that you need to use Lightroom CC 

https://helpx.adobe.com/lightroom-cc/how-to/lightroom-mobile.html