App.diagrams.net – How to Draw Self-Call in Sequence Diagram

The background pages are just drawn as you set shapes down on the respective areas. Just drop something outside of the page area, the new background will appear.

Select any/all edges that have the wrong rounding. On the right panel, under the style tab, there's a pull-down to select the edging stlye:

Select "Sharp" in that pull-down. Further down that panel is a "Set as default style" button that will set the current style for all new edges.