Does the Side Bar have to show by default?
No. A couple variables at the beginning of the script "Load Settings - On Startup --- Edit Configuration Here ---" can be set to make the "no Side Bar" layouts the defaults. We have a second variable for the calendar's side bar as users sometimes want that to show / hide differently than the main side bar.
A few tips:
The calendar was designed to run with the side bar showing by default so you might find it more interesting to add relevant content there than to remove it.
If you do set the side bar's default to hidden and decide to keep it that way, you'll likely want to visit File / Manage / Layouts and make the "no Side Bar" layout show up in the layout menu. This way the calendar will show you where you are in the layout menu and you'll reduce a but of screen flashing on Windows machines. You can even rename these layouts, switching their emphasis so "Projects" becomes "Projects - with Side Bar" and the no side bar layout just becomes "Projects".
How can I get at the tabs used here?
As you enter layout mode you'll see instructions in orange text about how to drag the tab covering away and reveal the native tabs below. This page and the video linked to there should help.