Adding new fields to shapefile alters appearance of borders in QGIS?

What is the role of snapping in creating new features in a spatial data layer in QGIS?

The Snapping group provides related options, namely handling of the snapping tolerance and the search radius. Snapping tolerance: When you add a new vertex or move an existing one, the snapping tolerance is the distance QGIS uses to search for the closest vertex or segment you are trying to connect to.
 

How do I add a border line in QGIS?

Option 1 – Quickest way

1. double click on the layer you want to style (in your case the border layer)
2. under Symbology select Hashed line.
3. play around with the settings. To obtain something similar to the example of the question, set Rotation to 45° (or -45° )

How do I stop editing in QGIS?

This can be found in the context menu after right clicking on the legend entry for a given layer. button from the digitizing toolbar to start or stop the editing mode.