how to focus on a scene or shape or object ?

By workspace, do you mean Composition Frame? 

I see several significant problems with your project.

First, your Composition Panel is set to 12.5% magnification. That means an object that iw 100 pixels wide will only be 12 pixels wide in the Comp Viewer. 

Second, the left hand upper part Composition is only 31 X 99 pixels. At 12.5% magnification, the whole comp would be less than 4 pixels wide in the comp viewer. I can't see anything if I create a comp that size and set the Magnification to 12.5%. If I set it to Fit, the magnification is 1100%. Your comp is way too small to work with for any standard video delivery.

Third, I have no idea what the position values are for any of the layers in your active composition. The preview in the Project Panel shows what looks like an arm with a shirtsleeve. I don't see any other layers, and I can't see where different parts of the arm would fit in the tiny comp.

Fourth, Your composition panel is set to Full resolution. It is almost always best to set that to Auto. 

If you change the Magnification ratio to Fit or 100%, you should be able to find your incredibly small in the Composition Panel. Figuring out where the other elements are is another problem. 

I also don't know where the Gray color comes from. If you have set your Composition Background Color to Gray, you must realize that you still don't have any pixels in the background. The Gray will render black with almost all video formats because transparent = black, and Composition Background Color does not create any pixels. It is still transparent.

I am also unsure why you pre-composed the arm parts and sequenced the layers; each layer is only one frame long.

Where did you get the workflow? What size is your main composition? What size is the artboard in the Illustrator file you are using?

As far as zooming into a layer's boundaries or centering a composition view on a layer's anchor point, there is no shortcut for that. Fitting a comp to the Composition Panel using the Magnification Ratio popup or keyboard shortcut (Ctrl/Cmnd + /) should show you all layers positioned inside the composition frame at that current time. If you select the Zoom tool (z) or center the mouse pointer over the appropriate layer and scroll or push in, you should get a close-up view framed the way you like. You are not going to be able to see anything if the composition frame size is 31 X 99 if the Magnification is set to 12.5%.

You should also be aware that any comp that you send to be rendered to any delivery format like h.264 MP4 must be an even number of pixels high and wide and should conform to standard video format frame sizes for best playback performance. All lossy compression formats (anything for delivery to the public) must have blocks of at least 4 pixels to compress color. If you send a comp with an odd number of rows or columns of pixels the encoder will make up the difference and either add a black line or degrade the image.

Ctrl/Cmnd + / will center the Composition frame in the Composition panel and fit it to the panel size.

Double-clicking on a work area in the toolbar will allow you to reset your workspace. If the current workspace is unavailable in the toolbar, you can also go to the Window/Workspace menu and reset your workspace. This will also center the composition frame in the Composition panel.