Hi,
BuddyPress pages are not ordinary WordPress pages. These pages should be blank, without any template assigned. BP pages exist only for internal purpose and to have their own slug (and the benefit of permalinks). The content is always dynamic and change depending the context.
That is also the reason why you shouldn’t use a “page builder” for them untill you’re very comfortable with code and design.
If you need a full width output or a sidebar, you adjust that in your theme settings or preferences. You can easely do that with WP’s Customizer. Or course reading your theme documentation can also help.
If it is not possible that way, i recommand you use a child-theme, and add him any necessary part you’re actually missing. But ensure first that these parts are really missing !
BuddyPress comes with a few CSS rules who normally fit with any correctly coded theme. This means that it actually can be used with almost any existing theme.
BuddyPress has no sidebar
BuddyPress can be viewed on any existing device
BuddyPress fit to any screen size
This means that you have to adjust your theme first, and not BP.
For more details, read from Codex
BuddyPress Theme Development
