If you want a global default that runs on all folders:
Obsidian > Preferences > Templater > Folder Templates > Add New > and then in the first field, enter a forward slash /, and in the second field, enter the Templater template you want to use.
This template will fire in all folders unless specific templates are already applied to specific folders. For example, I have a folder for books and a Templater template for notes that relate to books. If I right click on that folder and choose new note, Templater will create a new note with the book template.
I have a separate folder for book covers and it does not have a specific Templater template applied to it. If I right click on that folder and choose new note, Templater will create a new note with whatever template has been assigned as the global template.
(Just learned this today while looking at your latest question. It would probably have been a better answer to your original question. Oops. Sorry.)
Pressing CMD N without directly interacting with a specific folder will create a new note according to this setting:
Obsidian > Preferences > Files & Links > Default location for new notes.
I use an _inbox folder as it is an easy place to see recent files and decide if they should be sorted into other folders or deleted. Not everyone likes to use a dedicated folder in this way. The joy of having choice.