If you use Elementor PRO, everything is much simpler. This functionality was created only for people who use the free version of Elementor, because in PRO version functionality to create single templates is already available by default. You can find out how to do this here.
Single Type Templates are sets of sections, widgets and media content that will be displayed on the pages of single posts.
You can find them by going to your Template Library and selecting the Single template type.
In Theme Options > Page Templates tab, you can choose standard single templates for single posts, for example, you can choose Single Post Default Style globally for all single post pages. Or create your own single page template and select it as a standard one.
You can also choose the type of single post template individually for each post. To do this, go to the page editing and find the Post Options panel in the sidebar, in it you will find the selection box with all single type templates. By default, all posts have a Default Template value (global value from Theme Options).