Could ad suggestions be related to Siri?

That's not possible. Siri is always on to listen for "hey Siri" or "Siri" to be invoked. No information is shared nor does Siri suggest ads.

Nope. What is more likely is that the suggestion came from a Search in a web browser or through activities on Social Media. If you are logged into a Google account on your phone, then log out immediately. In my opinion Meta and Google apps/search are considered Malware just by the use of your information that they are selling to advertisers. Apple sells physical products for their revenue, and those companies make YOU the product that they sell.