That’s a good question. I wrote an article on affiliate links that you can find on my site. It goes over everything on where to add them on a blog, social media account, YouTube videos, etc. Hope that helps! And you don’t always need a blog to promote affiliate links, as you could do it on social media or other channels. However, I recommend starting your own blog as you need a consistent audience to generate affiliate revenue.
Affiliates decide within their affiliate programs which banners or ads they're going to place on their websites. This decision is based on their individual calculations of which company ads their website visitors are most likely to be interested in. They will also agree to affiliate programs based on which merchants have the best commission structure, although the structure is not usually considered very high-paying or profitable.