I am hoping this question isn't off-topic as it's mainly about what a specific type of food is made of rather than how to make it. Of course, I could always reframe the question as "How do I make hotdogs and bologna from scratch exactly as it's done by mainstream companies?"  So I want to know how to find out what, exactly, supermarket bologna and hotdogs are made of. When I was young, I was told that all the leftover parts -- organs included -- went into the slurry before being formed into the final product. Later, I was told that it was only the trimmings. I see trimmings as the more undesirable cuts of meat such as the parts I trim off myself before prepping. This includes fatty tissue, sinew, etc. I do not, however, consider organs to be trimmings.  I know that some brands -- perhaps the lower-shelf, bargain brands -- might use literally every leftover part, but what about mid- to top-shelf brands? In the middle, there are brands like Kahn's, Oscar Mayer, and Ballpark. Near the top of the shelf you have Hebrew National, Nathan's, and others. How can I tell which brands use organs and other unsavory leftovers and which use actual trimmings?  Also, is there a difference between the beef variety vs. the kind made with chicken and pork with beef added other than the type of animal they come from? Does the type of animal have an impact on what part of the animal goes into the mix? Is the beef variety more "premium" or do they all contain the same tier of meat trimmings? 
OP here answering my own question after doing some research.  First off, here's an excerpt from this USDA resource:  https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/meat-preparation/hot-dogs-and-food-safety/CT_Index     Byproducts, Variety Meats   "Frankfurter, Hot Dog, Wiener, or Bologna With Byproducts" or "With Variety Meats" are made according to the specifications for cooked and/or smoked sausages (see above), except they consist of not less than 15% of one or more kinds of raw skeletal muscle meat with raw meat byproducts. The byproducts (heart, kidney, or liver, for example) must be named with the derived species and be individually named in the ingredients statement.   So hot dogs and bologna can indeed contain organs, but they must be named on the packaging. Therefore, if you don't see heart, kidneys, brain, or other organs listed in the ingredients then it's safe to say the product is devoid of those parts. In this case, the product should consist entirely of skeletal meat, though this will be mostly trimmings.  Secondly, it also comes down to how the meat is removed. See below excerpt to this USDA resource:  https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/food-labeling/meat-and-poultry-labeling-terms/meat-and-poultry-labeling-terms     MECHANICALLY SEPARATED MEAT   is a paste-like and batter-like meat product produced by forcing bones with attached edible meat under high pressure through a sieve or similar device to separate the bone from the edible meat tissue. In 1982, a final rule published by FSIS on mechanically separated meat said it was safe and established a standard of identity for the food product. Some restrictions were made on how much can be used and the type of products in which it can be used. These restrictions were based on concerns for limited intake of certain components in MSM, like calcium. Due to FSIS regulations enacted in 2004 to protect consumers against Bovine Spongiform Encephalopathy, mechanically separated beef is considered inedible and is prohibited for use as human food. However, mechanically separated pork is permitted and must be labeled as "mechanically separated pork" in the ingredients statement.   So the difference between the beef and chicken and pork products is that chicken and pork can be mechanically separated, but beef cannot. What this means is that the chicken and pork product will contain whatever was on the bone at the time of separation which includes not only skeletal meat, but also skin, connective tissue, a regulated quantity of bone, and even blood vessels. That's not to say that the beef product is guaranteed to be devoid of these parts, but it's far less likely since the separation is manual and, therefore, more discriminate.  I think the differences between the separation processes also explain why the beef product is significantly more expensive since there's less automation to reduce costs.  Anyway, I will be marking this as answer in a few days unless anyone can point out any errors in my research. 