How protected is the inside of the phone, like the internals, against bumps and drops?

If you want to look at how an iPhone is put together inside there are people who have taken them apart and posted the results on the internet. There really is nothing inside (such as rubber) that guards against damage due to shock. Everything is packed in there like sardines. Some people who have dropped one without any glass or serious case damage have found a display that shows odd lines - possibly from the LCD connector being jarred. Some people don't see anything bad as a result of a drop, while others have it completely stop working.

The only real way to protect an iPhone is with an external case. Some are really heavy duty such as the Otter Box Defender.

I've dropped my iPhone 6 on hardwood flooring twice from about four feet without repercussions but that's just my experience. Nary a scratch.