I love hot dogs. With mustard and sauerkraut or grilled onions or chili. I love the taste. Especially the “dirty water” dogs served on the streets here in midtown. I’ve heard various accounts of what goes into these things. But I need to know for sure. Which animals and what parts of those animals? What is the outside “skin” made out of? Etc.
Trimmings (generally from decent cuts) are used for the filling.
Casings, when they exist, are intestine. Perfectly healthy.
mmmmm, lips and assholes!
Actually, here’s what you want:
As stated there, most of the hot dogs you buy in the store will have a synthetic casing, as will most sausages of any type you purchase at the supermarket.
You might as well lick the slaughterhouse floor. 
2 more questions:
1- It seems that hot dogs are comprised of an almost random selection of pork and beef trimmings. If that’s the case shouldn’t different varieties of them taste different. To me they all have a similar taste.
2- How are sausages different than hot dogs?