Okay, Thanks, I’ll look into it!
Ohhhh okayyyy, I thought the setting was for #4 not #2, I was confused, I understand now, thank you!
How do you apply them?
If by “apply” you mean “change” - you can customize the existing First and Default master pages however you like. In Engrave mode, show the panel on the right by pressing Ctrl/Cmd-9, then double-click the master page you want to edit - they’re in the section called Master Pages. For instance, you could add a project subtitle token to the text frame containing the project title, and format the text so that it’s centered and italic, say, or create a paragraph style for subtitles with that formatting.
Here’s an introduction video to master pages.
I mean like applying the Master Page to the Score itself. Okay, Thanks, I’ll watch it.
Layouts automatically use the First and Default master pages. First for, well, the first page of music 
and Default for subsequent pages. You can change when Dorico uses the First master page automatically in Layout Options.
For overrides, such as when you want to apply a custom master page to a specific page, see here.
For changing the master page set used for a particular layout, see here.
So would it be smart to create a new score?
To have a title, subtitle etc? No - just edit the master pages in this project to how you want them. If those changes aren’t appearing in the current score, remove any previous overrides (that prevent Dorico from updating pages).
Editing master pages only affects them in that project - not any others, unless you export the edited set and import it into other projects.
You can of course have any number of scores in a single project as you like - that’s the flexibility of Dorico.