How can I add Products to Opportunities?

I have added few products we sell and nopw I like to make those Products selectable (multiple selection) when new Opportunity is added.
I spent few hours with Studio but now luck because Opportunities > Fields > Add fields will not show me proper fields that get data from Products.

There is a product_category_dom and etc but those are not related to actual Products.

Can someone walk me through how to get this done.

You may either create one to many relationship between Opportunity and Product module, which will give you subpanel of product inside Opportunity.
If you want a dropdown field of product names in opportunity, then create a enum function type field and get the Product names as a multi select in Opportunity. Solution entirely depends on what you want to achieve with it.