Internal bleeding is really a layman’s term…most often it’s used to mean bleeding into the GI tract. This might present as vomiting blood if it’s from the stomach (usually an ulcer) or black stools if it’s from the stomach or just past the stomach (because stomach acid turns blood black) or just bloody stools if it’s lower in the GI tract or if there isn’t enough acid to turn the blood black.
GI bleeding is usually diagnosed with a long skinny fiberoptic endoscope. You can reach most of the places is the upper GI tract that bleed, as well as most of the colon. It’s unusual for the small intestine to bleed except from ulcers which are just past the opening from the stomach.
The blood does clot but with some bleeds an ulcer erodes a lot of the vessel wall. Some people have high pressure veins in the esophagus and upper GI tract from conditions such as cirrhosis…
It is possible but unusual to have bleeding that is literally internal. Things like a ruptured ovarian cyst that tears a vessel, or a ruptured spleen or a malformed vessel or a cancer in the chest or a big artery that develops an aneurysm and burst–all these things and much more can bleed freely into the chest and abdominal cavity. Pain and faintness are common symptoms and diagnostic techniques vary. On average a CT scan, for instance, can show blood, especially if a contrast medium is injected to help small amounts of blood show up more easily.
There isn’t a general rule that internal bleeding is more dangerous than external bleeding. I’ve had patients get into trouble or die from either. Since there are hundreds of causes, no particular rule applies except perhaps that “external” bleeding is more typically traumatic and easier to control if you know what to do.