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fragment has a nav_graph
nav_graph has a start_destinstion
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remember that a fragment can have a nav_graph(navigation)
and nav_graph will dictate the subfragments(e..g I click on fragment1's button and then I go to fragment2. I click on fragment2's button and then go back to fragment1)
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In fragmentContainerView, 0 height and 0 width means the fragmentContainerView will fill the entire activity
you can see how much the activity is filling from your design preview thing
To change it, change layout_width and layout_height