JS needed for WordPress modal for media uploads. Use wp_enqueue_media(); in your enqueue function to get the modal to show

custom-fields.js

jQuery( function( $ ) {
	"use strict";

	/**
	 * Open media library in modal
	 */
	function imageModal( metaBoxSelector ) {

		let frame,
			metaBox      = $( metaBoxSelector ),
			addImgLink   = metaBox.find('.upload-custom-img'),
			delImgLink   = metaBox.find('.delete-custom-img'),
			imgContainer = metaBox.find('.custom-img-container'),
			imgIdInput   = metaBox.find('.custom-img-id');

		// ADD IMAGE LINK
		addImgLink.on('click', function (e) {

			e.preventDefault();

			// If the media frame already exists, reopen it.
			if (frame) {
				frame.open();
				return;
			}

			// Create a new media frame
			frame = wp.media({
				title: 'Select or Upload Media Of Your Chosen Persuasion',
				button: {
					text: 'Use this media'
				},
				multiple: false  // Set to true to allow multiple files to be selected
			});

			frame.on('select', function () {

				let attachment = frame.state().get('selection').first().toJSON();

				imgContainer.append('<img src="' + attachment.url + '" alt="" style="max-width:100%;"/>');

				imgIdInput.val(attachment.id);

				addImgLink.addClass('hidden');

				delImgLink.removeClass('hidden');

			});

			frame.open();

		});

		// DELETE IMAGE LINK
		delImgLink.on('click', function (event) {

			event.preventDefault();

			imgContainer.html('');

			addImgLink.removeClass('hidden');

			delImgLink.addClass('hidden');

			imgIdInput.val('');

		});

	}

	// Image modal for Region cpt posts
	imageModal( $( '#region.postbox' ) );

	// Image modal for Licensee users
	imageModal( $( '.form-table' ) );

});

// custom fields library
https://gist.github.com/Magnacarter/ad8e2fa66fd2018017165ffe573a9067

// implement library
https://gist.github.com/Magnacarter/198ab650752f48850735f9478ced2eb0