WordPress CMS question: Landing pages as part of main site or separate?

I've got a quick question about using WordPress as your CMS.

Let's say you set up your main site in WP using a certain theme.

Now you want to also have a landing page for this site.

Are there any advantages / disadvantages of having the landing page live there on the same WP site?

I'm not concerned about how much room the landing page would add to the hosting plan.

I'm more concerned about how confusing it might be for a non-techie like me to keep from screwing up the main WP site or the landing age itself.

I know it's possible to have the landing page be a part of the main site and use a different URL which is redirected to that page of the main site (the landing page).

Or it could be off by itself, with its own URL.

Also, if you've got your main WP site set up with its header and footer, does that mean every page within that WP site must have that same header? Can you change the header on one or two pages of the site? Or does that kick me back out into making more sense to have those pages sitting off to the side as their own sites rather than being part of that main WP site?

Hope that makes sense.

Thanks!

Jill