Ask HN: How do Google ads work

Google always serve me with ads of websites I have visited before. For example I visited nexmo.com and since then, all the ads showing on the websites I visit are Nexmo.

Forgive me if I am missing some points here.

How I thought online advertising should work is that the advertising platform should be able to show me ads related to my interests. In a case of Nexmo example, I should be able to see others ads related to Nexmo.

I believe online advertising should be a way of helping a user discover useful information based on their interests.

I stand to be collected, I don't have any technical know-how on how online ads work BUT its not been useful to me.