Vinyl on shirts is Heat Transfer Vinyl- a thin layer of vinyl material on a clear plastic sheet that is cut using a plotter machine, then heat pressed onto a shirt. The vinyl we use for clothing is quality- it won’t easily peel off, fade, or crack no matter how many times you wash it.
For windows and such, Adhesive Vinyl is used- it has a paper backing, and is sticky to touch when the paper backing is peeled off. This is what makes it sticky. It is designed more for permanent outdoor application, but also comes in indoor types as well.