Custom page template – slug and title

Your question is confusing.
Template files are loaded according to their file name, and how that matches the Template Hierarchy. A template named page-about.php would be loaded for a Page with a slug of ‘about’.
Template parts can be used in any template file.
Templates that have a PHP comment at the top, in a certain format, are called “page templates” but shouldn’t use page- in the filename. They are listed in the editor for the user to choose to apply to that page or post, and if one is chosen, then loading the template by file name is bypassed.

The template file is only about displaying the content in the database. WordPress won’t load a template until after the content is retrieved. The template file has nothing to do with creating the content, so no, WordPress will not create your about page automatically.

Sorry for my confusing question.

It was more about which one should I use between page-about.php and page_about.php. However, your answer is helping me a bit to understand the difference.

As I underdand, the slug is the URL after .com/slug so page-about.php would be .com/about or .com/page-about ?

So if I create a page called “Part 1 of my adventure” for example, and decide to use the slug “about”, it would use the page-about.php template right? But it can only be used one time…

If I need to translate my page and use a plugin, would I need to use another template page since slug would be different? Would be better to use a global template then…

Thank you

If you create a page in WP titled “About”, its slug will typically be about. There might be other permalink elements in an URL, but the page’s slug is still just about. It’s also possible to change a page’s slug on the edit screen, but WP assigns a slug automatically based on the page title. However, it’ll be all lowercase and all spaces are replaced with -. If the slug already exists, a number will be appended.

You request the page by its permalink. The template file name itself is never directly requested. If a template file named "page-{$slug}.php" (substituting the actual slug for {$slug}) exists in the theme, that template file will be used to display that page’s content.

So, if you request example.com/about/ and such a page exists, and a template file named page-about.php also exists, then that file will be used to display the requested page. If the permalink were example.com/2021/05/about/ instead, the page slug is still just about and page-about.php will still be used.

If the permalink were example.com/2021/05/about/ instead, the page slug is still just about and page-about.php will still be used.

This part is incorrect. Pages don’t have permalinks like this, so that would not be a Page, and so that template file would not be a match for a post, according to the Template Hierarchy.

So if I create a page called “Part 1 of my adventure” for example, and decide to use the slug “about”, it would use the page-about.php template right? But it can only be used one time…

Yes, the slug is what is used, not the title, although WP uses the title to generate the slug, so if you want it different, you have to change it yourself.

If I need to translate my page and use a plugin, would I need to use another template page since slug would be different?

That’s a good question. I think the various plugins do things differently, but the address of the page is what WP uses to determine which content to retrieve and which template to use to display it. The plugin could store different languages and intercept each URL to swap out the content, or it could use different URLs for each language.

There’s not much to do in a page template to make things different. I recommend that you focus on content and ignore page templates. I wrote a theme and could think of only one reason to make a page template, which was for listing taxonomy terms.