https://en.wikipedia.org/wiki/Oatmeal

https://en.wikipedia.org/wiki/Porridge

Oatmeal is a more specific term than porridge, because porridge isn’t necessarily made from oats (eg, rice in Asia).

Though in the UK, when someone says “porridge” they almost always mean oat porridge or oatmeal.

"Oatmeal" is a very confusing word to me because it's not actually "meal" (meaning "flour" in this case). There is actually a flour made out of oats, but it's not what you would make porridge out of.

Oat porridge (aka oatmeal, aka oatmeal porridge) is one type of porridge.

Yes.