before you dive into anything, you should be familiar with some of the WordPress Concepts and Terms as Content Editor, just to make your life easier and more effective follow this guide to customize your website.
the Site Editor is the new interface since WordPress v5.8, to Customize the Dynamic Pages for Datatypes for example the Archive page for Posts or Services, or the main layout of the website and any details dynamic pages like the blog post itself, or the Service page etc., and it is based on what we Call Blocks or Components design system, small components that comes together to build something new.
you can find all Phenix Blocks in the Design Category of the Blocks List from the Editor.
the Styles and Colors of your Website theme, can be customized from the Site Editor Styles, when you select it, you will go to the editor with a style panel opened in the sidebar and from there you can edit the colors of the website following the next image.
the Templates in the Site Editor is where you can Customize your dynamic pages like Archive, Single Post page.
the Block Patterns are Pre-Designed Templates that you can add into the content and edit it as you need to, and it can be used across the Block Editor and the Site Editor, and another Type of Block Pattern is Synced Pattern, which will change everywhere like a Template Part.
the Template Parts however is Components that can be used in the Templates of the Site-Editor as shared parts of content like the Header or the Footer of your website that you can change from one place which is the Template Part.