Facebook doesn’t enable you to put mobile users in an audience out of the box. You will have to submit the data to the Facebook Pixel yourself, e.g. with parameters in the URL or by firing an event when a user is on a mobile device. Then you can base a custom audience on that event. https://facebook.com/business/help/964258670337005

>I want to remarket to people who added items into their cart but did not check out. Is it possible to create a custom audience with Shopify of people who abandoned their carts?

Again, I’d recommend fully setting up the Facebook Pixel so you can make use of the events “Add to cart” and “Purchase”.