Hello @marcraigraham
1. Do you have a Page configured as the “Posts Page” in Settings > Reading? If so, edit that page, and you can find a link to the “Edit template” of the page that shows all posts on it: https://d.pr/i/QkmkaW
Make sure you did not assign any template to that page before choosing it as your Posts page.
2. The same goes for single posts pages. The “Single” template used for posts displays the Featured Image in the header. You can easily use the site editor to remove the group with elements at the top and add a new “Template Part” block in which you can show a different Header layout. https://d.pr/i/JxZ4RI
I hope this helps. It would be helpful if you could provide some screenshots if this doesn’t help.