It's extremely up-front, the home page is offers for credit cards and loans and such. It's also explained here: https://www.creditkarma.com/faq/howitworks

In several years of using it, they've never emailed me advertising anything, and while they're obviously sharing my info with credit card companies and banks, I haven't gotten any marketing from them either. If you don't care about the recommendations, as I don't, you can use it for your taxes and never think about it the rest of the year.

They use your data (income, financial situation (do you have a mortgage, etc.)) to determine what financial products (which credit cards, personal loans, mortgages, etc.) to offer you.

This would all be in addition to what they knew about your credit score or anything else you input on the other parts of their platform.

But as others pointed out, that part of CK got split off when Intuit bought them.

“wonder”

The DOJ forced them to sell that whole unit if they wanted to be acquired because of the super obvious data mining consolidation

When I did my taxes through them, at the start they offered to transfer my data into Cash App. The wording was a bit shady as it wasn't clear what Cash App is or if it was part of the tax prep process or not. I already knew what Cash App is so I declined but I could see some people getting confused. At the end it also offered to load my refund into a Cash App account "in days instead of weeks", making it essentially a refund anticipation loan. The wording on that one was much more clear and I declined.

They tell you. Advertising dollars for financial products.