The Events Calendar (TEC) and Event Aggregator (EA): Link Site B's imported TEC events to their canonical Event Tickets location on Site A. Displays a big button on Site B's single events pages.

functions.php

<?php
/**
 * The Events Calendar (TEC) and Event Aggregator (EA): Link Site B's imported TEC events to their canonical Event
 * Tickets location on Site A. Displays a big button on Site B's single events pages.
 *
 * Useful when you want to import and display Site A's events (running TEC and ET/ET+) on Site B (running TEC and EA).
 * WARNING: Watch out if you're also importing from other sources, like Google, Eventbrite, etc.
 * Read this snippet's code comments to make sure you understand what this code does and what it does not do.
 *
 * @link https://gist.github.com/cliffordp/aab5ea85e9bd409d09cb8e22a3690a25 This snippet.
 * @link https://cl.ly/43705ea2b5d6 GIF of this snippet in action on Site B.
 * @link https://theeventscalendar.com/knowledgebase/creating-hub-site-event-aggregator/ Link Site B's events (running EA) to Site A's tickets.
 * @link https://cl.ly/6b07851ee7c7 Example import settings (for single event).
 * @link https://cl.ly/f22b7c37a0a7 View of Site B's database entry from Site A's imported event data.
 */
function tribe_tec_ea_hub_site_b_link_back_to_site_a_tickets() {
	$site_a_event_link = get_post_meta( get_the_ID(), Tribe__Events__Aggregator__Event::$source_key, true );

	// Might also want to check if this event doesn't have its own tickets.
	if (
		// Bail if no import source.
		empty( $site_a_event_link )
		// Bail if no cost was imported, which will be set on Site B if Site A event had a ticket.
		// Remove this logic if Site A adds its first ticket AFTER Site B already imported it... or manually set an Event Cost for Site B's event data.
		|| empty( tribe_get_cost() )
	) {
		return;
	}

	/**
	 * We just assume every Site A is a TEC site and that it has tickets.
	 * Might want to customize to add additional sanity checks to avoid bad linking...
	 * such as whitelisting the domain(s) of sites we import from...
	 * such as SiteA.com and SiteZ.com so we don't link to Eventbrite.com
	 *
	 * Hash:
	 * #buy-tickets     If Site A uses Event Tickets Plus (ET+) - WooCommerce or Easy Digital Downloads.
	 * #rsvp-now        If Site A uses RSVP.
	 * #tpp-buy-tickets If Site A uses Tribe PayPal Tickets.
	 *
	 * If Site A uses more than one of these ticket types, just drop the hash or leave it there (will work the same).
	 * Example URL: https://wpshindig.com/event/event-tickets-demo/#rsvp-now
	 */
	printf(
		'<div class="%s" style="text-align: center;">
			<a href="%s" target="_blank">
				<button class="tribe-button" style="padding: 40px; font-size: 200%%;">
					Check Ticket Availability
				</button>
			</a>
			<br>
			<small><em>
				(Link opens in a new window.)
			</em></small>
		</div>',
		__FUNCTION__,
		trailingslashit( $site_a_event_link ) . '#rsvp-now'
	);
}

/**
 * Top of event, after featured image.
 */
add_action( 'tribe_events_single_event_before_the_content', 'tribe_tec_ea_hub_site_b_link_back_to_site_a_tickets' );

/**
 * Bottom of event, after meta box(es).
 * FYI: Event Tickets hook into tribe_events_single_event_after_the_meta at Priority 4.
 * Relevant if Site B events might add their own tickets in addition.
 */
add_action( 'tribe_events_single_event_after_the_meta', 'tribe_tec_ea_hub_site_b_link_back_to_site_a_tickets' );