Edit Homepage middle section

Hi there,

I would like to make this section(Blog section bit wider)

- Try this option: https://docs.generatepress.com/article/container-width/

would like to restrict or display the blogs of a particular category only

- This should help: https://generatepress.com/forums/topic/multiple-static-frontpage-selected-posts-on-frontpage/#post-157558

Adding PHP: https://docs.generatepress.com/article/adding-php/

I would also like to get rid of GeneratePress reference link in the footer.

- This should help: https://docs.generatepress.com/article/changing-the-copyright-message/

thanks for this.

How can I include some introduction text before the blogs?

You can either use a header element:
https://docs.generatepress.com/article/header-element-overview/
https://docs.generatepress.com/article/page-hero-examples/
https://docs.generatepress.com/article/page-hero-examples/

Or Block element with after_header hook if you prefer the block editor:
https://docs.generatepress.com/article/block-element-overview/