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AndroidStudio Project consists of Modules, every project can have 1 or more modules. When you "Run" you run needed module not project.

An app consists of a window - an Activity. Each activity consists of one or several elements (buttons, check-boxes, radio buttons etc.), the Android name for this element is View (Button, TextView, ImageView etc.). To collect views into a group of views we use ViewGroup (LenearLayout, RelativeLayout etc.).

To create activity file for the other screen orientation:

left click on res => right click on res => New => Adnroid resource file => Available qualifiers => Orientation

Size of views in the ViewGroup:

1. actual size in dp/dpi (dot in one inch)
2. constant values - match_parent / wrap_content
3. weight: 
1. weight=0 equals to wrap_content
2. weight is the proportion of the view in the 100% of the overall parent ViewGroup. e.g.: button1 has weight=2, button2 has weight=3, button3 has weight=5. so 2+3+5=10 10=100% and 1=10% of the parent ViewGroup actual size.

ViewGroups:

1. LinearLayout can be horizontally or vertically oriented
2. RelativeLayout child views are organized relatively to parent or to each other
3. FrameLayout - reservers space needed to show a single view (useful with Fragments)