Understanding the scene collections and heirarcy

It just so happens that you can have one object in multiple collections. The object will be in one original collection and then you can basically link it to others. You must have used Shift + M at some point. 

There are a couple of things you can do.
1. Use multiple collection to organize your pieces. You can also select an object and press shift+g->collection->Then the collection name if the object is a member of multiple  collection. This will select everything in that collection. 
2. Use the [ and ] keys in 3D Viewport. If you have an object selected and press [ it will select the parent of the selected object if it has one. ] will select all children of the selected object. if you want to keep the original object selected then use shift+ [ or ]. Side note: you can use Shift+G-> Parent, Children, or Siblings if you want to select objects that have parent/child relation. To keep the original selected after you click, you will have to either expand adjustment box in bottom left corner of the 3D viewport or press F9 to bring it to the cursor and check extend.
3. Collection/Hidden collection instancing. First, basic collection. move all objects into a collection. click the check box next to the collection name this removes it from the view layers. So, it won’t render this one and everything in won’t be seen in the 3D viewport. Then with mouse cursor in the 3D viewport press shift+a->Collection Instance->then click the name of the collection you removed from view layers. This instance will move and act as one object. You can always active the collection and adjust the objects in it.(Move, rotate, scale, edit, sculpt, etc) Second, the more advance hidden collection. Instead of unchecking the view layer box you right click on the collection and click unlink. You can still add it with shift+a->collection instance->{Collection name}, but the collection no longer shows in the outliner. If you want to access the collection again you have to change the outliner mode(second icon on the header) to blend file. then scroll down to collections and expand it. Then right click on the collection name and click Link to Scene. Then you can change the mode back to View Layer and it is back. 

Don’t worry, it’s not that the Outliner is perfect, there are quirks, such is the Blender way. One convenient thing is that the “eye” icon is for hiding in the viewport:

So when you hit Alt + H to un-hide objects, things that have the icon turned off will pop again into view. But the monitor icon is for disable in the viewport, so it not only hide things, it disables them:

So if you have the monitor icon set to off, the objects that are this way, when you hit Alt + H to bring things back into view, the ones that have the monitor icon disabled wont come back into view, because they are disabled altogether.

Also if your end goal is to show or hide the head or body while you are working on other pieces, you can turn this logic inside out and not focus on hiding them, rather isolate the piece you want to work with. You can do that with Local View:

Hot Key is Numpad slash. So if you want to work on a foot and everything is getting in the way, instead of hiding everything else, just select the foot and isolate it to local view.

I see you found a solution. I’m going to add to it. The first 20 collections can be isolated in object mose. The order depends on how you have your collections ordered in the outliner. So top levels first. Then sub levels. The hotkeys are the numbers on the top row. 1 thru 9 and 0 for first 10. Alt+ 1-0 or 11 thru 20. Shift+ to toggle view instead of isolating. You can see the order and do the same thing through n-panel->view tab->collection panel. To isolate a collection click collection name. To toggle show hide click Eye icon. This combined with local(/) and show/hide(h, shift+h, alt+h) makes working on large scene a lot easier. Also when in local view. If you have an object that you want to remove from the local view you can select it and press m.