Modern page template option in UI of document libraries
Currently, you can only select a modern page template for a new page when you are on the homepage and select the New menu option for a page.
If you are in a pages library and select New -> Site Page, it doesn't give you the option to select a template
Andrew Goodwin commented
This is because the Site Pages library still uses Content Types. The other 'New' menus on the start page and from other pages don't, they use the new page templates functionality.
Reading between the lines, if MS haven't implemented a consistent 'New' menu option for pages in the Site Pages library it's maybe because content types for site pages are going to go away or change dramatically in how they are managed via the new Content Service admin center.
This is likely to be the best option in the short term for deploying page templates: https://mattipaukkonen.com/2020/07/02/distribute-sharepoint-page-templates-with-pnp-powershell-and-provisioning-templates/, then inform your users to not create pages from the library menu.
See also this UserVoice suggestion: https://sharepoint.uservoice.com/forums/329214-sites-and-collaboration/suggestions/41062459-make-modern-page-creation-experience-consistent
Creating a news site where there are multiple groups of people (i.e. Publishers) who each want to be able to have their own branding to allow for audiences to differentiate between different publishers, you would expect a product like SharePoint to be able to deliver this very necessary feature.
Brian V commented
This is really important, because there are times we want to create a page in a folder of the sitepages library. (Due to a few pages needing unique permissions). The only way to create a page in a folder is to use the library view, and that prevents the template selection screen from being used.
Agreed, it is odd that my content authors have to return to their home page in order to start a new page from a template.
When I found out that it is not possible, I was very surprised... it is so obvious that you want to access the templates in the library.
Why is there no way to apply this from the Site Pages or Add a page?