Create a custom taxonomy dropdown in WordPress

custom-taxonomy-dropdown.php

<div class="all-tools">

	<?php
		// If the taxonomy term has previously been selected from the dropdowm menu, grab it
		$term = isset( $_GET[ 'tools' ] ) ? sanitize_text_field( $_GET[ 'tools' ] ) : false;

		// In this section, set selected and taxonomy_query values before creating the taxonomy dropdown & getting WP_Query results
		$selected = '';
		$tax_query = '';
		if ( $term ) {

			// Get all posts with the selected taxonomy term
			$term = get_term_by( 'slug', $term, 'tools' );
			if ( ! empty( $term->name ) ) {

				// Set the 'selected' value that we'll use to show which option in the dropdown menu is the currently selected one
				$selected = $term->name;

				// Set the taxonomy to 'tools' and the term to the selected term. We'll use these to filter the posts, below
				$tax_query = array(
					array(
						'taxonomy' => 'tools',
						'terms'    => $term,
					),
				);
			}
		}
	?>

	<form id="tool-category-select" class="tool-category-select" action="<?php the_permalink(); ?>" method="get">
		<?php
			// Create and display the dropdown menu using 'tools' for the taxonomy, and the selected value we set above
			wp_dropdown_categories(
				array(
					'orderby'         => 'NAME',		// Order the items in the dropdown menu by their name
					'taxonomy'        => 'tools',		// Only include posts with the taxonomy of 'tools'
					'name'            => 'tools',		// This field is needed for submitting the form
					'value_field'     => 'slug',		// This field is needed for submitting the form
					'show_option_all' => 'All Tools',	// Text the dropdown will display when none of the options have been selected
					'selected'        => $selected,		// Set which option in the dropdown menu is the currently selected one
			) );
		?>
		<input type="submit" name="submit" value="view" />
	</form>

</div>

<section id="tools-listing">

	<?php
		// Get all posts with a post type of 'tool', a taxonomy of 'tools', and the taxonomy term that was selected in the dropdown
		$tool_query = new WP_Query(
			array(
				'post_type'      => 'tool',
				'posts_per_page' => 1000,
				'tax_query'      => $tax_query,
			)
		);
	?>

	<!-- Loop through every post that matched our query -->
	<?php if ( $tool_query->have_posts() ) : while ( $tool_query->have_posts() ) : $tool_query->the_post(); ?>
		
		<!-- Display each post's title and content - you can change this section to display whatever post content you want -->
		<article class="tool-entry">
			<h1><?php the_title(); ?></h1>
			<p><?php the_content(); ?></p>
		</article>

	<?php endwhile; endif; ?>

</section>