Help adding custom fields in admin page

I am working on my own theme and I want to add fields in admin page that allow the user to set images, such as banner image, callout1, callout2 from the admin page.

I got this idea after using the Athena theme and I noticed they had options to set various images and text using custom fields in the dashboard. these were not widgets. These were completely custom fields which appeared under appearance->customize and the fields to change slider image and various text areas was included there.

If anyone could at least point me in the right direction that would be helpful because right now, I don’t even know what to search for. Maybe Meta Boxes??