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.
Current estimated total network hashrate is 387 EHS which is 387*10^18 hashes per second.