For an online business, this can be quite simple. Correctly set up, your marketing partner will deliver reports which tell how many of your ads lead to a website visit and a sale or booking.
Whether you Facebook, Instagram, TicToc, Google Ads, or some other service, you will get detailed information on: Where the people who clicked on your ad live, which page of your website they visited. Then, your sales pages will know: did they buy anything, did they abandon the sales cart, etc.
Ideally, you want to collect permission to send Emails directly to your customers.