This video shows how to use Opacity Masks in Adobe Illustrator to add texture to your t-shirt designs, logos, and lettering. It also covers how to export your design as a transparent PNG, which is a preferred file format for Direct-to-Garment (DTG) printing, Print-on-Demand (POD), and Amazon Merch.
Resources discussed
In case you missed it, here's a video showing how to use textures, layer masks, and brushes in Adobe Photoshop.