All company or products have keywords. Keywords are those by which people search on Google. If there is a keyword of a product on your website, then your website will show related ads of the same keywords. When Google’s Robots visit your blog and detect a keyword in your website, they match it with AdWords and show the same ads to their products.
If you have written about smartphone in your blog, then advertisement related to smartphone will appear in your blog. All these ads belong to those companies that have put real keywords from them in their products in Google Ads.