Hey Leslye,
I will cover the questions in the same order:
1). This is not theme-dependent, as content is separated from presentation in WordPress; you can set up the blog posts page in the „Settings -> Reading” admin section, and it can be added as a menu item in the „Appearance -> Menus” admin section. See e.g. this guide for more details.
2). Not sure I understood the question – could you please clarify? The three layouts you mention determine how a blog post will look like, i.e. they are layouts for the blog post content type. Might I be missing the point here?
3). The widgets you add to the „Sidebar” widget area in the „Appearance -> Sidebars” will appear on each page or post which has a right-sidebar or left-sidebar layout, as well as the blog posts page, which always has a sidebar by design in Ikebana. When you choose a blog posts page as mentioned in the first point above, it will display all widgets added to the „Sidebar” widgets area automatically; each individual post will display the same widgets as well, unless you give it a „full-width” layout.
I know WP can seem convoluted at times, I hope that makes it clearer!
Sincerely,
Andrii / Satori Studio