Yoast WordPress SEO, question about meta description template for pages

I’m somewhat new to wordpress and Yoast WordPress SEO, but I have read many articles carefully and followed setup instructions and guides. My question is regarding:

In Dashboard>Titles & Metas>Post Types — does adding data in the fields ‘meta description template’ for posts, pages, media and sliders REPLACE what may have already been entered when I wrote the pages and added data in the fields when I created the pages, or get ADDED in ADDITION to those created when writing the pages?
I have read multiple articles and setup instructions, but it’s not clear to me how this works in relationship to the SEO data fields when creating a new page. (https://managewp.com/google-seo-by-yoast-guide)

Currently, I have only the default settings in this section – Titles & Metas>Post Types – but have filled out the focus keyword, seo title and meta description in the wordpress SEO box on each ‘add page’ when creating or editing the page. Is this the best practice, or should I be changing the setting in the dashboard?

The website I’m working on is: liveonthecoastofmaine.com

Thanks

https://www.ads-software.com/plugins/wordpress-seo/