Are You think about the custom posts for develop or add in theme..ANd you get stuck on it. Here You get your Easy solution without using any plugin.
By default, WordPress comes with these post types:
1.Post
2.Page
3.Attachment
4.Revision
5.Nav Menu
You can create your own custom post types and call them whatever you want.

Follow this step:

First, go to your directory then go to function.php.Then open with any editor.
Note: Must apply this in your child theme. If you have no child create a child theme and then use the child theme.

function create_posttype() {

register_post_type( ‘name’,
array(
‘labels’ => array(
‘name’ => __( ‘NAME’ ),
‘singular_name’ => __( ‘Name’ )
),
‘public’ => true,
‘has_archive’ => true,
‘rewrite’ => array(‘slug’ => ‘name’),
‘show_in_rest’ => true,
‘menu_icon’ => ‘dashicons-admin-page’,

)
);
}
add_action( ‘init’, ‘create_posttype’ );

In Name, You can choose any name what name you want to use.