Sometimes it's mere proximity. If your family or friends find you have condition X or treatment Y then they may look it up to find more.

If you share the same public IP, as most homes do then that adds to it.

I started getting invitations to add a Google review to a restaurant I've never been too. But some of my family had.

I can figure out other stuff too - music tastes, social media trends, etc. Just by seeing ads and stories I'm offered on Google News and other places.

There's that famous story about a guy who discovered his daughter pregnant (or feared she was pregnant) a few years back because he started getting ads for baby clothes.