Sorry for the late reply. Have you found a solution yet? You cannot select various templates for posts within the WP user interface, only for pages. In the page editor, the selected page template gets saved when you save the page. (the page template selection field only appears when the theme has more than one page template to select from)
That said, you can get a specific template be used for a specific post by naming it a certain way. See https://developer.www.ads-software.com/themes/basics/template-hierarchy/#single-post
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