For each post you can select from available post templates:
- Default template: full width post template.
- Left Sidebar: post template with left sidebar. Widgets are added to Blog/Post Sidebar in Appearance/Widgets.
- Right Sidebar: post template with right sidebar. Widgets added to Blog/Post Sidebar in Appearance/Widgets.
Tip: You can bulk edit all posts to apply page template to all or selected posts at once: https://bobwp.com/bulk-edit-posts-wordpress/