Are Iphones' content encrypted?
The iPhone is built with encryption inserted automatically when writing to flash memory and decryption inserted automatically when reading from it. This happens with all files, all the time. There is no concept of "decrypting a file and then opening the decrypted file", as there would be on a PC.

No file can be decrypted without the proper key. Some files (such as the ones needed to start up the phone) are decrypted with a key that is always known to the encryption/decryption machinery. Other files are decrypted with a key that is only provided to the machinery once the phone has been unlocked.

That is why unlocking the phone is fast. All that happens is that an additional key is made available to the decryption machinery, so that it is able to decrypt files encrypted with that key.

All this is an over-simplification, since there are more levels of keys than this. The absolutely authoritative source, which is also remarkably easy to read, is Apple's own iOS Security Guide.