How to change home page?

Also installing a new theme with a custom home page, in my site still remain the default home, how can I set the new home page based on them?

Again, i'm a bit confused, you are asking about setting different themes for different pages on your site? Your homepage specifically using a unique theme?

First Grav does not natively support multiple themes, but there is a 3rd party plugin called Themer that can achieve this.

Alternatively you can simply create a new 'homepage' twig template in your main theme that is styled very differently to achieve the same results.

oh no no man, I wanna use the same them for the entire site (that's really matter) but I see that Grav support different kind of pages, but in the admin panel I cant create those pages, that's why Im learning how to create new advanced pages (listing, modular) via my code editor.
In this case I need to learn a bit more about contant and decalrations in Grav.
Thank you very much again, I'll be present more and more here... I just started yesterday night in using Grav :)