Is there anywhere to find more specific information about difficulty-- eg. What is the current target value for candidate block hashes?
There is a difference between difficulty and the target.
Difficulty is only used when we want to represent "how hard" it is to mine a block. Current difficulty is found here:
https://blockchair.com/bitcoin/ and is calculated as explained here:
https://en.bitcoin.it/wiki/DifficultyTarget on the other hand is the actual value used in mining to determine the suitable header hash and is explained here:
https://en.bitcoin.it/wiki/TargetIt's value is found in each block's header in a short 32-bit form that can be converted to the long 256-bit form.
Or, on average, roughly how many nonce values need to be tested (in the billions? quadrillions?) to find such a value?
On average it depends on the hashrate of your device which is the number of hashes it can compute per second.