This is a very very broad question, probably isn’t the place for it tbh. But anyway, there are many many themes in existence already, you can modify them, generally by adding a child theme and making your modifications there, it is bad practice to update a theme directly.
If you wish to extend the functionality of your site, then maybe a plugin, custom or not may be a better solution for you. If you wish to add custom post types, meta fields etc, it is advisable to do this outside of the theme core files.
Again there are a vast array of plugins out there to extend and modify the look, functionality and usability of practically any type of site!
Start here – https://codex.wordpress.org/Theme_Development