Newbie trying to learn custom post templates

We’ve setup 3 post categories. Parish Papers, News and Bulletins. The Parish Paper posts require a different header than the other two categories. I’d like to use this opportunity to learn how to create a custom post template but I’m not sure where to begin. Is there a free and reliable plugin I can use to layout a custom post template for Parish Papers? Is this ability already baked in to Gutenberg? Here’s our setup: Blocksy Theme, Gutenberg with Blocksy Companion (free). I’ve wandered around reviewing different tutorials and still a bit lost. If anyone can point me in the right direction, I’d appreciate it.

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