Hello Ahmed,

Editing the theme is fairly easy: you have the portfolio items and normal pages with options and content. The most important portfolio options are the hero image and captions (title and subtitles). The content can be edited as for normal  pages, nothing special about it.

So the header you are talking about is the hero section that can be changed in page/portfolio item options -> hero section tab. in wp admin -> portfolio you Edit (not Edit with Elementor) and when you scroll past the editor you are able to see the options panel.

You don't actually edit it. the overlapping gallery is selecting the portfolio items from admin -> portfolio.

you can configure how many items to include and a category filter.