First to file means that, as someone who invented earlier but did not file, you aren't going to be able to get that the patent reassigned to you. But you can still use your prior art to get the patent invalidated.
> ...as someone who invented earlier but did not file, you aren't going to be able to get that the patent reassigned to you.
There's an exception: When a second applicant disclosed the invention publicly both prior to the first application and less than one year prior to the second application.
Another note: Prior art needs to be publicly available. So, a first inventor's private notes can't invalidate a second inventor's patent. In the old first-to-invent system, however, the private notes of a first inventor could allow the first inventor to get the patent ahead of a second inventor who filed first.
If you invent something and someone else patents it, you show prior art to invalidate the patent.
Whether you used software or a lathe or a lawnmower or excel to make the thing doesn’t matter.