Yes, I completely agree with you! I think being able to read through the actual code of the software you use, in order to fully understand it, what it does, and how it works is a priceless tool. On top of that, most open source tools also provide live examples of implementations, which can be extremely helpful.

However, what do you say to those who will argue that it's actually the other way around? They'll say that private companies have full developments teams paid and dedicated towards maintaining the security of their products... meaning that more work and research gets done, making them more secure, and trustworthy in the end. Or those who say that open sourcing the code makes it easier to find vulnerabilities?