Regarding the question you asked - How does Instagram know what I google
I'm figuring that the foundational knowledge about this topic is slowly being lost to time, and there are new course modules in schools around the world to cover this type of thing, so I think it's appropriate to cover it here. There is a separation between the content of a site and the advertisements shown on the site. Sites like Instagram (or newspapers, blogs, or any content providing site) show their own content, but then leave blank spaces to be used by advertising networks. Ad networks then fill these blank spots on the pages that you see, but the content provider (Instagram) may not ever see this information (nor do they care). Ads are not the content of the content provider.
So, what happened has nothing to do with what was in your WhatsApp conversation, but everything to do with the Google search you did.
You searched for the business using Google while you were logged into Google on your desktop and your phone. This created an entry in the Google ad networks that you are interested, right now, about this business. When you opened Instagram, the ad network knew what you were interested in, and showed you an ad based on your recent search.
So, nothing was leaked or shared. A blank field in a template was filled by the ad network based on your activity seen by the ad network. You could go to a variety of other content sites and see the same ad.
This is very relevant to the topic of "security" due to the growing public understanding of "information warfare". Ads can be used to target people's beliefs and ideologies and show specific ads to specific people and can make it look like that it is content from a trusted source (like a newspaper (yes, I am speaking idealistically that journalists can be trusted at a meta level, but let's not get sidetracked by a debate around that)). But the newspaper/blog did not create the content, and it is created, specifically, to engage the person on a specific issue to shift the person's thinking in a specific way that benefits the paying content provider. So, it is important to understand and identify when content is from an outside 3rd party and how it gets into your feeds so that you can evaluate the trustworthiness and bias of the source.
