(Sometimes it’s referred to as offal, sound to me as waste (in Dutch: afval), what it is not.)

The kidneys are simply useful. You may possibly cut the tough channels out.
ganzenleverThe liver is a delicacy. If he is very pale or mottled, than rather put it off. Watch it, the bile is also on / in the liver. A small bag with a very dark green bitter liquid. If you spill it over meat or liver, it is virtually unusable. So prefer to cut something of the liver, instead of breaking the bile.
The same caution is necessary for the bladder. Which will usually be (almost) empty. They were used as a tobacco pouch, football,...
The stomach is only a problem if you have the inputs and outputs cuts. Put that out of the carcass, so the gunk is gone. There are useful pouches and water bags to make of it. In chickens, the stomach is a firm and tasty muscle. You should peeled the tough inside.
The heart is tough. It is a muscle that always works (once).
Lungs were also edible. I always put them away. Dog food. Ditto for eyes. You can certainly not store lungs.
Udder (breaded cutlets) and testicles are quite tasty. But only tell after eating what was served. The problem is between our ears.
Organ meats better can be used quite quickly. The shelf life is short. Therefore, it was a slaughter feast: for a short time, there was an abundance of quickly to process residues. The Brussels triperie (tripe) and ‘Choesels au Madere’ (in Madera sauce) are an example.
Brains and tongue are a delicacy. For who loves it.
Intestine can be turned inside out to scrub them clean with the back of a knife. You can use them for sausage, string, rope, condoms...

Before cutting you leave the meat in a cool place overnight to stiffen up. Just as it is cool enough.

Haggis is a typical Scottish dish, a sheep's stomach or beef intestine, stuffed with pieces of heart, lung, liver, kidney and oatmeal and to your taste onion, pepper, salt, nutmeg and mace.
The dish reflects the classic Scottish economy: use everything.
In our region, all the less attractive and tough cuts of the pig are processed in brawn (especially meat of the head) and head chees (rind (skin), legs, tail).
Casings are here used to run (black pudding, white pudding...) all sorts of sausages.

“Waiter, I tasted no sausage in the sausage sandwich!”“Then you just missed it, sir!”(Max Tailler)
No, thank you. No ox tongue for me. I do not want anything that comes from the mouth of a beast. Give me rather an egg.