As mentioned earlier, 'by' is used when we are talking about means, mechanism, or the agency of something. Have a look:

We use 'through' when we are talking about systems and the process of something. Take a look at the following examples:

These two prepositions are used interchangeably and that is perhaps the reason that learners confuse them. They are interchangeable in the favor of flow of speech and the user's personal definition of mechanism and system.