How to work with Sensitive (personal) Data, without actually seeing it?
You can use the following techniques to mask sensitive data:

Substitution cipher -  any character of plain text from the given fixed set of characters is substituted by some other character from the same set depending on a key.

Tokenization masking - mask source string data based on criteria that you specify in an algorithm.

Principal Components Analysis (PCA) or other dimension-reducing techniques - combine several features and then carry out ML training only on the resulting PCA vectors.

Data Coarsening - decrease the precision or granularity of data to make it more difficult to identify sensitive data within the dataset.


Also, read  Data Masking on Wikipedia.