General histology questions, what are the differences between these layers?

Mucosa = Includes both epithelia and lamina propria. It is the surface (mucousy) portion. The lamina propria is the loose CT directly underneath the surface epithelium and above the basement membrane.

Submucosa = Connective tissue that joins the surface mucosa with the muscle layer underneath.

Endothelium = is the same thing as simp squam epithelium. It is just more specific. This term is used when describing the internal linings of blood vessels.

Not 100% sure on this one but serous vs adventitia.... Both are outermost layer of CT. Serous (hence the name) obviously secretes serous fluid.

As i mentioned submucosa is underneath the lamina propria. They are not the same.

Agree with most of the other answer. Little knicknacks: as the other guy stated, endothelium is essentially the inner epithelial lining of blood/lymphatic vessels. You are not supposed to *call* it epithelium though. You are supposed to call it endothelium. It's silly wordplay, but that is the classic convention (not necessarily strictly adhered to anymore).

Adventitia is a more general term than serosa. Adventitia is the connective tissue that is on the outermost part of the organ in question. Serosa refers specifically to the adventitia of the intestinal tract that is exposed to the peritoneal cavity.