Isolation Forest Prediction Mechanics: Does it compare value with every tree (and the original training subset)?
You can find the sklearn implementation of Isolation Forest in Python at https://github.com/scikit-learn/scikit-learn/blob/a24c8b46/sklearn/ensemble/iforest.py#L229

It calculates the mean path depth needed to classify new samples. They are scored relatively to a theoretical average path length. Original training data is not used, only the learned trees.