How do they use bias to sell their products? How would you use skepticism to evaluate the data that these companies use in their advertisements?

They use cognitive biases like framing, loss aversion, status quo etc. to elicit an emotional response in hopes of you making impulsive purchases.

You can use skepticism by listening more carefully to what is being said and looking up any claims made by the advertiser. Like if they say they are the cheapest car insurance, you don't just believe them you compare prices.