Ask HN: How to explain common people what online tracking is?

Every now and then, someone in my social circle wonders how they have been talking about some product or service with someone, only to see an ad for it a bit later in their social media feed.

Then they wonder how this happened. Paranoia kicks in like "this phone is listening to us" or "they can read our encrypted chat messages".

I've tried to find ways to explain this topic in simple, layman's terms, but mostly failed by going too much into details and confused them even more.

What's equally important is that I'd like to give them actionable tips so they can protect themselves and their friends & family.

What's your go-to explanation on this topic, and the best (simple) actionable advice on how they can protect themselves?