What Python skills for a fraud detective?

[snippsat]

I already learned some basic Python skills. I would like to learn python skills for searching through a lot of files. What should i focus on? Should i use books or a online course?

For this should look into os module and pathlib(a more modern way).
To give example let say want to find a .txt file and that the search is recursively(all sub-folders) for this file.

from pathlib import Path

dest = r'C:\Test'
for path in Path(dest).rglob('*.txt'):
    if path.is_file():
        print(path)

The same with os.walk.

import os

dest = r'C:\Test'
for root, dirs, files in os.walk(dest):
    for file in files:
        if '.txt' in file:
            print(os.path.join(root, file))

I would say that looking most into pathlib(Taming the File System) is the way to go.
Also learn command line tools like find, grep, awk ...ect