Any league’s official website is the place to look for that kind of information, but since Opta is the EPL’s official stats provider, there shouldn’t be any discrepancies between the two.
The EPL’s fantasy league has a much broader range of what they will credit as an assist. For example, a shot saved by the keeper and tapped in by a third player will count as an assist in fantasy league, but won’t be officially counted as an assist by the league. (Mane got an assist in fantasy for Firminho’s goal). Another example is penalties - fantasy gives assist points for winning a penalty, but that doesn’t really count as an assist in the real world. The same applies for own goals I guess.