If you want to keep the left sidebar in Blog page then you can select via customize inside the Basic setting which helps you to show the sidebar in the Blog page and if you want to show the sidebar in the pages then you need to select the page template which you will get inside the page or you can follow the following steps to select the page template:
Go to Dashboard => Pages => select the page where you want to show the sidebar while selecting the page you can get edit option then click in the edit => after you enter the page you can get the option to select the page template in right sidebar => Then select the page template and update it.
And regarding the widget in customize we haven’t kept it all so, you can use it via Appearance => widget.
Please let us know if we are understanding you fully or if perhaps this is not what you meant.
We look forward to your response