This is an example plugin showing how to register custom post types and custom taxonomies

frammenti.php

<?php
/**
 * @package frammenti
 * @version 1.0
 *
 * Plugin Name: Frammenti
 * Plugin URI: http://wordpress.org/extend/plugins/#
 * Description: This is a development plugin 
 * Author: Carlo Daniele
 * Version: 1.0
 * Text Domain: frammenti
 * Domain Path: /languages/
 * Author URI: https://frammentidicodice.com/
 */

function frammenti_plugin_init() {

	$labels = array(
		'name'			=> _x( 'Knowledgebase', 'post type general name', 'frammenti' ),
		'singular_name'		=> _x( 'KB article', 'post type singular name', 'frammenti' ),
		'menu_name'		=> _x( 'Knowledgebase', 'admin menu', 'frammenti' ),
		'name_admin_bar'	=> __( 'KB article', 'frammenti' ),
		'add_new'		=> __( 'Add New KB', 'frammenti' ),
		'add_new_item'		=> __( 'Add New KB', 'frammenti' ),
		'new_item'		=> __( 'New KB', 'frammenti' ),
		'edit_item'		=> __( 'Edit KB', 'frammenti' ),
		'view_item'		=> __( 'View KB', 'frammenti' ),
		'all_items'		=> __( 'All KB articles', 'frammenti' ),
		'search_items'		=> __( 'Search KB articles', 'frammenti' ),
		'parent_item_colon'	=> __( 'Parent KB:', 'frammenti' ),
		'not_found'		=> __( 'No KB found.', 'frammenti' ),
		'not_found_in_trash'	=> __( 'No KB found in Trash.', 'frammenti' )
	);

	$args = array(
		'labels'		=> $labels,
		'description'		=> __( 'Description.', 'frammenti' ),
		'public'		=> true,
		'publicly_queryable'	=> true,
		'show_ui'		=> true,
		'show_in_menu'		=> true,
		'query_var'		=> true,
		'rewrite'		=> array( 'slug' => 'kb' ),
		'show_in_rest'		=> true,
		'capability_type'	=> 'post',
		'taxonomies'		=> array( 'section', 'topic' ),
		'has_archive'		=> true,
		'hierarchical'		=> false,
		'menu_position'		=> null,
		'supports'		=> array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'custom-fields', 'revisions' )
	);

	register_post_type( 'knowledgebase', $args );

	$section_labels = array(
		'name'			=> __( 'Sections', 'frammenti' ),
		'singular_name'		=> __( 'Section', 'frammenti' ),
		'search_items'		=> __( 'Search Sections', 'frammenti' ),
		'all_items'		=> __( 'All Sections', 'frammenti' ),
		'parent_item'		=> __( 'Parent Section', 'frammenti' ),
		'parent_item_colon'	=> __( 'Parent Section:', 'frammenti' ),
		'edit_item'		=> __( 'Edit Section', 'frammenti' ),
		'update_item'		=> __( 'Update Section', 'frammenti' ),
		'add_new_item'		=> __( 'Add New Section', 'frammenti' ),
		'new_item_name'		=> __( 'New Section Name', 'frammenti' ),
		'menu_name'		=> __( 'Sections', 'frammenti' )
	);
	
	$section_args = array(
		'hierarchical'		=> true,
		'labels'		=> $section_labels,
		'show_ui'		=> true,
		'show_admin_column'	=> true,
		'query_var'		=> __( 'section', 'frammenti' ),
		'show_in_rest'		=> true,
		'rewrite'		=> array( 'slug' => __( 'section', 'frammenti' ) ),
	);

	register_taxonomy( 'section', 'knowledgebase', $section_args );

	$topic_labels = array(
		'name'		=> __( 'Topics', 'frammenti' ),
		'singular_name'	=> __( 'Topic', 'frammenti' ),
		'search_items'	=> __( 'Search Topics', 'frammenti' ),
		'all_items'	=> __( 'All Topics', 'frammenti' ),
		'edit_item'	=> __( 'Edit Topic', 'frammenti' ),
		'update_item'	=> __( 'Update Topic', 'frammenti' ),
		'add_new_item'	=> __( 'Add New Topic', 'frammenti' ),
		'new_item_name'	=> __( 'New Topic Name', 'frammenti' ),
		'menu_name'	=> __( 'Topics', 'frammenti' )
	);
	
	$topic_args = array(
		'hierarchical'		=> false,
		'labels'		=> $topic_labels,
		'show_ui'		=> true,
		'show_admin_column'	=> true,
		'query_var'		=> __( 'topic', 'frammenti' ),
		'show_in_rest'		=> true,
		'rewrite'		=> array( 'slug' => __( 'topic', 'frammenti' ) ),
	);

	register_taxonomy( 'topic', 'knowledgebase', $topic_args );
	
}
add_action( 'init', 'frammenti_plugin_init' );

function frammenti_load_textdomain(){
	load_plugin_textdomain( 'frammenti', false, basename( dirname( __FILE__ ) ) . '/languages/' ); 
}
add_action('plugins_loaded', 'frammenti_load_textdomain');