a layout viewport is a window that displays a view of model space on a layout tab. You can create multiple layout viewports to display different views of your model at different scales.

To create the viewports in a layout tab, you can use three methods:

• From the Viewports dialog box: select a viewport configuration
• Create individual viewports by using the MVIEW command
• From the View Manager dialog box, generate named views and add them as layout viewpoints

These methods will allow you to create and arrange viewports on your layout tab.

https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2018/ENU/AutoCAD-Core/files/GUID-2B5D404A-DCAB-4AF6-A5C1-51593B38F519-htm.html

 object snaps are modes that allow you to snap to specific points on objects. To snap to the centroid of a closed polyline, you should use the Geometric Center object snap mode. This mode will display an asterisk (*) marker when you move the cursor over the geometric center of a closed polyline or spline.

To use this mode, you can either:

• Click on the Object Snap icon in the Status bar and select Geometric Center from the menu.
• Type GCE at the command prompt or when prompted for a point and press Enter.

Now you can snap to the centroid of a closed polyline by using Geometric Center object snap.

polar tracking is a feature that allows you to draw lines at specific angles. You can configure polar tracking to work at any angle you want by following these steps:

• Click on the arrow next to the Polar Tracking icon in the Status bar and select Settings.
• In the Drafting Settings dialog box, click on the Polar Tracking tab.
• Under Additional Angles, click on New and enter 6.25 as the angle value. Click OK.
• Make sure that 6.25 is checked in the list of additional angles. Click OK.

Now you can draw a line at an angle of 6.25 degrees by using polar tracking.

From the Delta option of the Lengthen command, enter 50 units and then select the right end of the line. This way, you can increase the length of the line by 50 units from its current endpoint without changing its direction or position2

https://www.learnvern.com/autocad-tutorial-in-hindi/how-to-use-lengthen-command