Pages or posts?

Hi,

I have 2 things but don’t know if to post here in one topic or two.

1. I know that when creating a new page, WordPress will automatically appear in a new tab (don’t know if WordPress 5 also does that).
How can I make it appear as a button in the main page, and if I press the button it will open and showo the page?

2. When clicking on a button , there’s sometimes a warning popup.
What is the best way to use for the popup, a page or a post?