How to edit the Template (of a theme?)

I’m making a Website with DreamPress. My theme is Twenty-Nineteen (I don’t think it matters), and I’m trying to edit the homepage.

This question is just a last-ditch effort for whatever.
I know the site has to be customized in the live editor, for titles/menus/headers and footers - and the pages have to be made in the builder with “blocks”. What I don’t understand is the “Template” for a page, and how to edit it without using code, or just getting rid of something I put there before.

I have made a home page for the site and everything worked together correctly. However, at some point whilst I was playing with WP/DreamHost to figure everything out, I added a form to the “Home Page” - which apparently, changed the template. Now every time I select the “Cover” template for a page, it adds this Form and I can’t delete it (using Pages, the Live Site Editor, or theme).

What even is this, man?

I am also facing such types of issues. I am also wanted to customize my website and add some custom sections. How it will be possible to add custom sections according to requirements on a theme page? There is any customizer or page editor used to customize?