The mucosa is the innermost layer, and functions in absorption and secretion. It is composed of epithelium cells and thin connective tissue. The mucosa contains specialized goblet cells that secrete sticky mucus throughout the GI tract. Read more about it through this article: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/22%3A_Digestive_System/22.05%3A_Layers_of_the_Alimentary_Canal/22.5A%3A_Mucosa#:~:text=The%20mucosa%20is%20the%20innermost%20layer%2C%20and%20functions%20in%20absorption,mucus%20throughout%20the%20GI%20tract.

The submucosa is a connective tissue layer deep to and supporting the mucosa. Examples: The substance of the submucosa is ordinary loose connective tissue. It allows the mucosa to move flexibly during peristalsis. Read it again here: https://histology.siu.edu/erg/giguide.htm#:~:text=The%20submucosa%20is%20a%20connective,to%20move%20flexibly%20during%20peristalsis.

the serous membrane (or serosa) is a smooth membrane that consists of a thin connective tissue layer and a thin layer of cells that secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid to reduce friction from muscle movements. Find more about this digestive tract tissue here: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/22%3A_Digestive_System/22.05%3A_Layers_of_the_Alimentary_Canal/22.5D%3A_Serosa