The encryption key is created from the passcode you create to lock the iDevice. The longer the passcode, the more encrypted the iDevice disk will be. I would doubt very highly that the passcode for your device is in any way linked with your Apple ID. Using the same passcode on your iDevice as for your Apple ID is probably not recommended.

A wealth of information is found on this page at Operational Security, with some pertinent info I've copy/pasted here.

If you own an up-to-date iPhone, iPad, or iPod, you are already running full disk encryption. iOS devices ship from the factory with non-user configurable encryption – “non-user configurable” means you can’t turn it off even if you want to.

Custom Alphanumeric Code: This passcode option provides the strongest security of all. This option should be considered if security is your primary goal. It should also be considered for some other scenarios: if you leave your device unattended, or if your device is at high risk of loss, theft, or capture. A custom alphanumeric passcode should also be used if you use Touch ID to unlock your iOS device and only rarely enter the passcode. This option has one significant downside: it requires the passcode to be entered on the full alphanumeric keyboard. This tiny keyboard offers the most complexity, but is incredibly tedious to work with, especially when you are in a hurry.

You can make a custom alphanumeric passcode even more secure by using some special characters on the iOS keyboard. The letters A, C, E, I, L, N, O, S, U, Y, and Z all contain special characters. For instance, the letter “a” contains the following special characters: à, á, â, ä, æ, ã, å, and ā. To access them, press the desired letter and hold. A pop-up menu will appear. Slide your finger to the desired character and release. Because of the immensity of the iOS keyboard’s character set, incredibly complex passcodes are possible.

iOS full disk encryption is only as strong as the passcode you use to protect it, so choose a good one. I recommend a six-digit numeric, but only if you can’t tolerate anything longer. If you can, I say go with a 10 to 12-digit numeric.

The encryption key is created from the passcode you create to lock the iDevice. The longer and more complex the passcode, the more difficult it will be to decrypt the iDevice disk. I would doubt very highly that the passcode for your device is in any way linked with your Apple ID. Using the same passcode on your iDevice as for your Apple ID is probably not recommended.

A wealth of information is found on this page at Operational Security, with some pertinent info I've copy/pasted here.

If you own an up-to-date iPhone, iPad, or iPod, you are already running full disk encryption. iOS devices ship from the factory with non-user configurable encryption – “non-user configurable” means you can’t turn it off even if you want to.

Custom Alphanumeric Code: This passcode option provides the strongest security of all. This option should be considered if security is your primary goal. It should also be considered for some other scenarios: if you leave your device unattended, or if your device is at high risk of loss, theft, or capture. A custom alphanumeric passcode should also be used if you use Touch ID to unlock your iOS device and only rarely enter the passcode. This option has one significant downside: it requires the passcode to be entered on the full alphanumeric keyboard. This tiny keyboard offers the most complexity, but is incredibly tedious to work with, especially when you are in a hurry.

You can make a custom alphanumeric passcode even more secure by using some special characters on the iOS keyboard. The letters A, C, E, I, L, N, O, S, U, Y, and Z all contain special characters. For instance, the letter “a” contains the following special characters: à, á, â, ä, æ, ã, å, and ā. To access them, press the desired letter and hold. A pop-up menu will appear. Slide your finger to the desired character and release. Because of the immensity of the iOS keyboard’s character set, incredibly complex passcodes are possible.

iOS full disk encryption is only as strong as the passcode you use to protect it, so choose a good one. I recommend a six-digit numeric, but only if you can’t tolerate anything longer. If you can, I say go with a 10 to 12-digit numeric.