WordPress can be used in a variety of ways, so there’s no one answer as to how you can do things. In many cases there are multiple ways to achieve the same goal. But, there are common uses:
Non-technical users often install WordPress and then install and configure premade plugins and themes. Themes control the look and feel of a site, while plugins add or change functionality. Usually little to no coding experience is required to use these. In many cases, you can find a way to achieve your goals without having to code anything.
More technical users may want to customize further. If this is the case, it’s usually helpful to first try premade themes and plugins to see what they do, and then gradually ease into development. Yes, you can use GitHub – and you can also use FTP or your host’s file manager, and a variety of other ways. FTP/SFTP is a very common scenario but not the only one available. Part of it depends too on what your host allows. Right now, there are several major types of code you may wish to explore – themes are primarily made of PHP and CSS; plugins are now split between PHP and JavaScript. That is because WordPress’s new editor, the Block Editor, uses primarily JS to create the blocks that you use to build out content.
In any case, any content you want to publish is created by logging into your site and adding or editing Posts, Pages, or custom post types. Posts and Pages come standard with WP Core; some themes and plugins add custom post types, and you can also create your own custom post types. Again typically it’s best to get used to the Core post types first, and then you can build things out further if needed. In addition to publishing content, many themes also support things like custom menus and widgets. You may just want to start playing around, and possibly create a development site – not the live one, but a local copy – to experiment and see what’s possible and what you may want to do.
WordPress.org documentation is a good place to start.