Hi amircn,

Thanks for reaching to us.

1, 2. You set this page as Portfolio Archive page, not just Portfolio, are usually non-editable via Page Builders or add content to it, inherit settings from Page Options as well. You can read more here https://codex.wordpress.org/Creating_an_Archive_Index

3. With Portfolio archive, in order to set styles, I answered here https://xtemos.com/forums/topic/portfolio-edit-page-design/#post-544022

For normal pages, you can use Portfolio widget, with more options for displaying https://prnt.sc/gkRLHiIB-7Hs

Regards,