How to make an optional variable product available on product pages and add it to cart using a radio button

Question

I want to make a companion product available on all product pages. I would like the user to be able to add this product to the cart by checking a radio button and in that same product the user would have the option to choose variations.

I would like to offer on all products an option for the person to subscribe to a discount plan. Example: 2 month plan $8, 6 month plan $12... how do that? Thanks!!!

Example:print choice plan fiverr