As a user you can search for tours based on different criteria such as tour type, duration, price, etc. Some tours can be booked immediately. You choose a departure date, book it immediately and pay online.
Some tours are request-based. In other words, you send your request for the tour provider and wait for the provider to confirm your booking or otherwise decline it. In both cases, you are informed and if the tour is confirmed, a payment link will be sent to you.