Opening the url http://my-sage-project.3000/portfolio it should use a specific template (for example template-portfolio.blade.php) but without creating a page from WP admin. It has to be just a code thing, without touching the DB
This is possible in Sage, because it’s possible in WordPress.
Since this is more about WordPress than about Sage, we recommend giving Stack Overflow a search, or posting your question there. You’ll probably get more thorough help on a question like this!
Hm, Sage 9 used the soberwp router which allowed customizable routing in WordPress.
Does Sage 10/acorn got a similar mechanism or is there a package that can be included?
OTOH shouldn’t this routing logic rather end up in a site plugin than in the theme?
Specific mechanisms for routing requests are largely beyond the scope of Sage as a framework, especially Sage 9. As @MWDelaney says, WordPress can do it, so you can do it with Sage, but walking you through the process is outside the scope of this forum.
There are a number of ways you can accomplish what you may be trying to do. I would recommend reading the following: