Widgets are normally managed in Appearance > Widgets, not the customizer.
As for displaying sidebar in specific pages, you have two places to set that up.
Globally by page type in
Customizing ▸ Layout > Sidebars & Widget Areas
where you can select if and how to display sidebars in any type of page.
you can then override the page type setting in any individual page by going in the page editing page > Xtreme options for this page > Layout TAB > Sidebar and widget drop down list