We are working in Sitecore 10.3 SXA. Is there any way to set the Page Design in __Standard Values for a page type template?
Or is there any other way to set the Page Design automatically once an author creates a page with a page type template?
Or is there any way to write a query in __Standard Values instead of the predefined tokens?
You should not use the standard values to achieve this. You can define the page design per template in the "Page Designs" item. You have a section "Designing" there and that can be used to set the default page design per template.
You can set the page design for the templates by navigating to Page Design item under the presentation folder as below.
Once you configure above mapping, whenever you create a page from the page template, page design will be automatically applied on the page.
Moreover you can also set page design using the rules on the Page Design item. Order of precedence of the page design will be as follows.
You can find more details here.
Hope this helps!!!
Using standard values for this purpose is not recommended. Instead, you can specify the page design for each template in the 'Page Designs' item. Within that item, there is a 'Designing' section that allows you to set the default page design for each template.
Alternatively, you can do it in the Experience Editor. Please see the two links for instructions on how to do this in both the Content Editor and the Experience Editor:
