How to add content in Left Sidebar and Right Sidebar

Content in Left Sidebar and Right Sidebar can be added via block regions.

Navigate to:
Home Administration Structure Block layout

example.com/admin/structure/block

Create a new Custom block or place any existing block in Sidebar Left or Sidebar Right block region.

In Visibility section of the block, you can configure on which pages this block will appear and which user roles can see this block.