Custom pages

For a theme, each template file should conform to the Template Hierarchy for general use. Read about page templates here: https://developer.wordpress.org/themes/template-files-section/page-template-files/
But a theme should use a different naming for the file if it is to be used for assignment to any page. It should not start with page- or it will be used as in the hierarchy. It must contain the page template comment at the top in order to show in the list of Page Templates to choose from.
Your file called page-mycustom.php would be used for a page with a slug of mycustom.